1 The Acorn Electron ULA
2 ======================
3
4 Principal Design and Feature Constraints
5 ----------------------------------------
6
7 The features of the ULA are limited by the amount of time and resources that
8 can be allocated to each activity necessary to support such features given the
9 fundamental obligations of the unit. Maintaining a screen display based on the
10 contents of RAM itself requires the ULA to have exclusive access to such
11 hardware resources for a significant period of time. Whilst other elements of
12 the ULA can in principle run in parallel with this activity, they cannot also
13 access the RAM. Consequently, other features that might use the RAM must
14 accept a reduced allocation of that resource in comparison to a hypothetical
15 architecture where concurrent RAM access is possible.
16
17 Thus, the principal constraint for many features is bandwidth. The duration of
18 access to hardware resources is one aspect of this; the rate at which such
19 resources can be accessed is another. For example, the RAM is not fast enough
20 to support access more frequently than one byte per 2MHz cycle, and for screen
21 modes involving 80 bytes of screen data per scanline, there are no free cycles
22 for anything other than the production of pixel output during the active
23 scanline periods.
24
25 Timing
26 ------
27
28 According to 15.3.2 in the Advanced User Guide, there are 312 scanlines, 256
29 of which are used to generate pixel data. At 50Hz, this means that 128 cycles
30 are spent on each scanline (2000000 cycles / 50 = 40000 cycles; 40000 cycles /
31 312 ~= 128 cycles). This is consistent with the observation that each scanline
32 requires at most 80 bytes of data, and that the ULA is apparently busy for 40
33 out of 64 microseconds in each scanline.
34
35 (In fact, since the ULA is seeking to provide an image for an interlaced
36 625-line display, there are in fact two "fields" involved, one providing 312
37 scanlines and one providing 313 scanlines. See below for a description of the
38 video system.)
39
40 Access to RAM involves accessing four 64Kb dynamic RAM devices (IC4 to IC7,
41 each providing two bits of each byte) using two cycles within the 500ns period
42 of the 2MHz clock to complete each access operation. Since the CPU and ULA
43 have to take turns in accessing the RAM in MODE 4, 5 and 6, the CPU must
44 effectively run at 1MHz (since every other 500ns period involves the ULA
45 accessing RAM). The CPU is driven by an external clock (IC8) whose 16MHz
46 frequency is divided by the ULA (IC1) depending on the screen mode in use.
47
48 Each 16MHz cycle is approximately 62.5ns. To access the memory, the following
49 patterns corresponding to 16MHz cycles are required:
50
51 Time (ns): 0-------------- 500------------ ...
52 2 MHz cycle: 0 1 ...
53 16 MHz cycle: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 ...
54 ~RAS: 0 1 0 1 ...
55 ~CAS: 0 1 0 1 0 1 0 1 ...
56 A B C A B C ...
57 F S F S ...
58 a b c a b c ...
59
60 Here, "A" and "B" respectively indicate the row and first column addresses
61 being latched into the RAM (on a negative edge for ~RAS and ~CAS
62 respectively), and "C" indicates the second column address being latched into
63 the RAM. Presumably, the first and second half-bytes can be read at "F" and
64 "S" respectively, and the row and column addresses must be made available at
65 "a" and "b" (and "c") respectively at the latest.
66
67 The TM4164EC4-15 has a row address access time of 150ns (maximum) and a column
68 address access time of 90ns (maximum), which appears to mean that
69 approximately two 16MHz cycles after the row address is latched, and one and a
70 half cycles after the column address is latched, the data becomes available.
71
72 Note that the Service Manual refers to the negative edge of RAS and CAS, but
73 the datasheet for the similar TM4164EC4 product shows latching on the negative
74 edge of ~RAS and ~CAS. It is possible that the Service Manual also intended to
75 communicate the latter behaviour. In the TM4164EC4 datasheet, it appears that
76 "page mode" provides the appropriate behaviour for that particular product.
77
78 The CPU, when accessing the RAM alone, apparently does not make use of the
79 vacated "slot" that the ULA would otherwise use (when interleaving accesses in
80 MODE 4, 5 and 6). It only employs a full 2MHz access frequency to memory when
81 accessing ROM (and potentially sideways RAM).
82
83 See: Acorn Electron Advanced User Guide
84 See: Acorn Electron Service Manual
85 http://acorn.chriswhy.co.uk/docs/Acorn/Manuals/Acorn_ElectronSM.pdf
86 See: http://mdfs.net/Docs/Comp/Electron/Techinfo.htm
87 See: http://stardot.org.uk/forums/viewtopic.php?p=120438#p120438
88
89 Bandwidth Figures
90 -----------------
91
92 Using an observation of 128 2MHz cycles per scanline, 256 active lines and 312
93 total lines, with 80 cycles occurring in the active periods of display
94 scanlines, the following bandwidth calculations can be performed:
95
96 Total theoretical maximum:
97 128 cycles * 312 lines
98 = 39936 bytes
99
100 MODE 0, 1, 2:
101 ULA: 80 cycles * 256 lines
102 = 20480 bytes
103 CPU: 48 cycles / 2 * 256 lines
104 + 128 cycles / 2 * (312 - 256) lines
105 = 9728 bytes
106
107 MODE 3:
108 ULA: 80 cycles * 24 rows * 8 lines
109 = 15360 bytes
110 CPU: 48 cycles / 2 * 24 rows * 8 lines
111 + 128 cycles / 2 * (312 - (24 rows * 8 lines))
112 = 12288 bytes
113
114 MODE 4, 5:
115 ULA: 40 cycles * 256 lines
116 = 10240 bytes
117 CPU: (40 cycles + 48 cycles / 2) * 256 lines
118 + 128 cycles / 2 * (312 - 256) lines
119 = 19968 bytes
120
121 MODE 6:
122 ULA: 40 cycles * 24 rows * 8 lines
123 = 7680 bytes
124 CPU: (40 cycles + 48 cycles / 2) * 24 rows * 8 lines
125 + 128 cycles / 2 * (312 - (24 rows * 8 lines))
126 = 19968 bytes
127
128 Here, the division of 2 for CPU accesses is performed to indicate that the CPU
129 only uses every other access opportunity even in uncontended periods. See the
130 2MHz RAM Access enhancement below for bandwidth calculations that consider
131 this limitation removed.
132
133 Video Timing
134 ------------
135
136 According to 8.7 in the Service Manual, and the PAL Wikipedia page,
137 approximately 4.7µs is used for the sync pulse, 5.7µs for the "back porch"
138 (including the "colour burst"), and 1.65µs for the "front porch", totalling
139 12.05µs and thus leaving 51.95µs for the active video signal for each
140 scanline. As the Service Manual suggests in the oscilloscope traces, the
141 display information is transmitted more or less centred within the active
142 video period since the ULA will only be providing pixel data for 40µs in each
143 scanline.
144
145 Each 62.5ns cycle happens to correspond to 64µs divided by 1024, meaning that
146 each scanline can be divided into 1024 cycles, although only 640 at most are
147 actively used to provide pixel data. Pixel data production should only occur
148 within a certain period on each scanline, approximately 262 cycles after the
149 start of hsync:
150
151 active video period = 51.95µs
152 pixel data period = 40µs
153 total silent period = 51.95µs - 40µs = 11.95µs
154 silent periods (before and after) = 11.95µs / 2 = 5.975µs
155 hsync and back porch period = 4.7µs + 5.7µs = 10.4µs
156 time before pixel data period = 10.4µs + 5.975µs = 16.375µs
157 pixel data period start cycle = 16.375µs / 62.5ns = 262
158
159 By choosing a number divisible by 8, the RAM access mechanism can be
160 synchronised with the pixel production. Thus, 264 is a more appropriate start
161 cycle.
162
163 The "vertical blanking period", meaning the period before picture information
164 in each field is 25 lines out of 312 (or 313) and thus lasts for 1.6ms. Of
165 this, 2.5 lines occur before the vsync (field sync) which also lasts for 2.5
166 lines. Thus, the first visible scanline on the first field of a frame occurs
167 half way through the 23rd scanline period measured from the start of vsync:
168
169 10 20 23
170 Line in frame: 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
171 Line from 1: 0 22 3
172 Line on screen: .:::::VVVVV::::: 12233445566
173 |_________________________________________________|
174 25 line vertical blanking period
175
176 In the second field of a frame, the first visible scanline coincides with the
177 24th scanline period measured from the start of line 313 in the frame:
178
179 310 336
180 Line in frame: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
181 Line from 313: 0 23 4
182 Line on screen: 88:::::VVVVV:::: 11223344
183 288 | |
184 |_________________________________________________|
185 25 line vertical blanking period
186
187 In order to consider only full lines, we might consider the start of each
188 frame to occur 23 lines after the start of vsync.
189
190 Again, it is likely that pixel data production should only occur on scanlines
191 within a certain period on each frame. The "625/50" document indicates that
192 only a certain region is "safe" to use, suggesting a vertically centred region
193 with approximately 15 blank lines above and below the picture. Thus, the start
194 of the picture could be chosen as 38 lines after the start of vsync.
195
196 See: http://en.wikipedia.org/wiki/PAL
197 See: http://en.wikipedia.org/wiki/Analog_television#Structure_of_a_video_signal
198 See: The 625/50 PAL Video Signal and TV Compatible Graphics Modes
199 http://lipas.uwasa.fi/~f76998/video/modes/
200 See: PAL TV timing and voltages
201 http://www.retroleum.co.uk/electronics-articles/pal-tv-timing-and-voltages/
202 See: Line Standards
203 http://www.pembers.freeserve.co.uk/World-TV-Standards/Line-Standards.html
204
205 RAM Integrated Circuits
206 -----------------------
207
208 Unicorn Electronics appears to offer 4164 RAM chips (as well as 6502 series
209 CPUs such as the 6502, 6502A, 6502B and 65C02). These 4164 devices are
210 available in 100ns (4164-100), 120ns (4164-120) and 150ns (4164-150) variants,
211 have 16 pins and address 65536 bits through a 1-bit wide channel. Similarly,
212 ByteDelight.com sell 4164 devices primarily for the ZX Spectrum.
213
214 The documentation for the Electron mentions 4164-15 RAM chips for IC4-7, and
215 the Samsung-produced KM41464 series is apparently equivalent to the Texas
216 Instruments 4164 chips presumably used in the Electron.
217
218 The TM4164EC4 series combines 4 64K x 1b units into a single package and
219 appears similar to the TM4164EA4 featured on the Electron's circuit diagram
220 (in the Advanced User Guide but not the Service Manual), and it also has 22
221 pins providing 3 additional inputs and 3 additional outputs over the 16 pins
222 of the individual 4164-15 modules, presumably allowing concurrent access to
223 the packaged memory units.
224
225 As far as currently available replacements are concerned, the NTE4164 is a
226 potential candidate: according to the Vetco Electronics entry, it is
227 supposedly a replacement for the TMS4164-15 amongst many other parts. Similar
228 parts include the NTE2164 and the NTE6664, both of which appear to have
229 largely the same performance and connection characteristics. Meanwhile, the
230 NTE21256 appears to be a 16-pin replacement with four times the capacity that
231 maintains the single data input and output pins. Using the NTE21256 as a
232 replacement for all ICs combined would be difficult because of the single bit
233 output.
234
235 Another device equivalent to the 4164-15 appears to be available under the
236 code 41662 from Jameco Electronics as the Siemens HYB 4164-2. The Jameco Web
237 site lists data sheets for other devices on the same page, but these are
238 different and actually appear to be provided under the 41574 product code (but
239 are listed under 41464-10) and appear to be replacements for the TM4164EC4:
240 the Samsung KM41464A-15 and NEC µPD41464 employ 18 pins, eliminating 4 pins by
241 employing 4 pins for both input and output.
242
243 Pins I/O pins Row access Column access
244 ---- -------- ---------- -------------
245 TM4164EC4 22 4 + 4 150ns (15) 90ns (15)
246 KM41464AP 18 4 150ns (15) 75ns (15)
247 NTE21256 16 1 + 1 150ns 75ns
248 HYB 4164-2 16 1 + 1 150ns 100ns
249 µPD41464 18 4 120ns (12) 60ns (12)
250
251 See: TM4164EC4 65,536 by 4-Bit Dynamic RAM Module
252 http://www.datasheetarchive.com/dl/Datasheets-112/DSAP0051030.pdf
253 See: Dynamic RAMS
254 http://www.unicornelectronics.com/IC/DYNAMIC.html
255 See: New old stock 8x 4164 chips
256 http://www.bytedelight.com/?product=8x-4164-chips-new-old-stock
257 See: KM4164B 64K x 1 Bit Dynamic RAM with Page Mode
258 http://images.ihscontent.net/vipimages/VipMasterIC/IC/SAMS/SAMSD020/SAMSD020-45.pdf
259 See: NTE2164 Integrated Circuit 65,536 X 1 Bit Dynamic Random Access Memory
260 http://www.vetco.net/catalog/product_info.php?products_id=2806
261 See: NTE4164 - IC-NMOS 64K DRAM 150NS
262 http://www.vetco.net/catalog/product_info.php?products_id=3680
263 See: NTE21256 - IC-256K DRAM 150NS
264 http://www.vetco.net/catalog/product_info.php?products_id=2799
265 See: NTE21256 262,144-Bit Dynamic Random Access Memory (DRAM)
266 http://www.nteinc.com/specs/21000to21999/pdf/nte21256.pdf
267 See: NTE6664 - IC-MOS 64K DRAM 150NS
268 http://www.vetco.net/catalog/product_info.php?products_id=5213
269 See: NTE6664 Integrated Circuit 64K-Bit Dynamic RAM
270 http://www.nteinc.com/specs/6600to6699/pdf/nte6664.pdf
271 See: 4164-150: MAJOR BRANDS
272 http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_41662_-1
273 See: HYB 4164-1, HYB 4164-2, HYB 4164-3 65,536-Bit Dynamic Random Access Memory (RAM)
274 http://www.jameco.com/Jameco/Products/ProdDS/41662SIEMENS.pdf
275 See: KM41464A NMOS DRAM 64K x 4 Bit Dynamic RAM with Page Mode
276 http://www.jameco.com/Jameco/Products/ProdDS/41662SAM.pdf
277 See: NEC µ41464 65,536 x 4-Bit Dynamic NMOS RAM
278 http://www.jameco.com/Jameco/Products/ProdDS/41662NEC.pdf
279 See: 41464-10: MAJOR BRANDS
280 http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_41574_-1
281
282 Interrupts
283 ----------
284
285 The ULA generates IRQs (maskable interrupts) according to certain conditions
286 and these conditions are controlled by location &FE00:
287
288 * Vertical sync (bottom of displayed screen)
289 * 50MHz real time clock
290 * Transmit data empty
291 * Receive data full
292 * High tone detect
293
294 The ULA is also used to clear interrupt conditions through location &FE05. Of
295 particular significance is bit 7, which must be set if an NMI (non-maskable
296 interrupt) has occurred and has thus suspended ULA access to memory, restoring
297 the normal function of the ULA.
298
299 ROM Paging
300 ----------
301
302 Accessing different ROMs involves bits 0 to 3 of &FE05. Some special ROM
303 mappings exist:
304
305 8 keyboard
306 9 keyboard (duplicate)
307 10 BASIC ROM
308 11 BASIC ROM (duplicate)
309
310 Paging in a ROM involves the following procedure:
311
312 1. Assert ROM page enable (bit 3) together with a ROM number n in bits 0 to
313 2, corresponding to ROM number 8+n, such that one of ROMs 12 to 15 is
314 selected.
315 2. Where a ROM numbered from 0 to 7 is to be selected, set bit 3 to zero
316 whilst writing the desired ROM number n in bits 0 to 2.
317
318 See: http://stardot.org.uk/forums/viewtopic.php?p=136686#p136686
319
320 Shadow/Expanded Memory
321 ----------------------
322
323 The Electron exposes all sixteen address lines and all eight data lines
324 through the expansion bus. Using such lines, it is possible to provide
325 additional memory - typically sideways ROM and RAM - on expansion cards and
326 through cartridges, although the official cartridge specification provides
327 fewer address lines and only seeks to provide access to memory in 16K units.
328
329 Various modifications and upgrades were developed to offer "turbo"
330 capabilities to the Electron, permitting the CPU to access a separate 8K of
331 RAM at 2MHz, presumably preventing access to the low 8K of RAM accessible via
332 the ULA through additional logic. However, an enhanced ULA might support
333 independent CPU access to memory over the expansion bus by allowing itself to
334 be discharged from providing access to memory, potentially for a range of
335 addresses, and for the CPU to communicate with external memory uninterrupted.
336
337 Sideways RAM/ROM and Upper Memory Access
338 ----------------------------------------
339
340 Although the ULA controls the CPU clock, effectively slowing or stopping the
341 CPU when the ULA needs to access screen memory, it is apparently able to allow
342 the CPU to access addresses of &8000 and above - the upper region of memory -
343 at 2MHz independently of any access to RAM that the ULA might be performing,
344 only blocking the CPU if it attempts to access addresses of &7FFF and below
345 during any ULA memory access - the lower region of memory - by stopping or
346 stalling its clock.
347
348 Thus, the ULA remains aware of the level of the A15 line, only inhibiting the
349 CPU clock if the line goes low, when the CPU is attempting to access the lower
350 region of memory.
351
352 Hardware Scrolling (and Enhancement)
353 ------------------------------------
354
355 On the standard ULA, &FE02 and &FE03 map to a 9 significant bits address with
356 the least significant 5 bits being zero, thus limiting the scrolling
357 resolution to 64 bytes. An enhanced ULA could support a resolution of 2 bytes
358 using the same layout of these addresses.
359
360 |--&FE02--------------| |--&FE03--------------|
361 XX XX 14 13 12 11 10 09 08 07 06 XX XX XX XX XX
362
363 XX 14 13 12 11 10 09 08 07 06 05 04 03 02 01 XX
364
365 Arguably, a resolution of 8 bytes is more useful, since the mapping of screen
366 memory to pixel locations is character oriented. A change in 8 bytes would
367 permit a horizontal scrolling resolution of 2 pixels in MODE 2, 4 pixels in
368 MODE 1 and 5, and 8 pixels in MODE 0, 3 and 6. This resolution is actually
369 observed on the BBC Micro (see 18.11.2 in the BBC Microcomputer Advanced User
370 Guide).
371
372 One argument for a 2 byte resolution is smooth vertical scrolling. A pitfall
373 of changing the screen address by 2 bytes is the change in the number of lines
374 from the initial and final character rows that need reading by the ULA, which
375 would need to maintain this state information (although this is a relatively
376 trivial change). Another pitfall is the complication that might be introduced
377 to software writing bitmaps of character height to the screen.
378
379 See: http://pastraiser.com/computers/acornelectron/acornelectron.html
380
381 Enhancement: Mode Layouts
382 -------------------------
383
384 Merely changing the screen memory mappings in order to have Archimedes-style
385 row-oriented screen addresses (instead of character-oriented addresses) could
386 be done for the existing modes, but this might not be sufficiently beneficial,
387 especially since accessing regions of the screen would involve incrementing
388 pointers by amounts that are inconvenient on an 8-bit CPU.
389
390 However, instead of using a Archimedes-style mapping, column-oriented screen
391 addresses could be more feasibly employed: incrementing the address would
392 reference the vertical screen location below the currently-referenced location
393 (just as occurs within characters using the existing ULA); instead of
394 returning to the top of the character row and referencing the next horizontal
395 location after eight bytes, the address would reference the next character row
396 and continue to reference locations downwards over the height of the screen
397 until reaching the bottom; at the bottom, the next location would be the next
398 horizontal location at the top of the screen.
399
400 In other words, the memory layout for the screen would resemble the following
401 (for MODE 2):
402
403 &3000 &3100 ... &7F00
404 &3001 &3101
405 ... ...
406 &3007
407 &3008
408 ...
409 ... ...
410 &30FF ... &7FFF
411
412 Since there are 256 pixel rows, each column of locations would be addressable
413 using the low byte of the address. Meanwhile, the high byte would be
414 incremented to address different columns. Thus, addressing screen locations
415 would become a lot more convenient and potentially much more efficient for
416 certain kinds of graphical output.
417
418 One potential complication with this simplified addressing scheme arises with
419 hardware scrolling. Vertical hardware scrolling by one pixel row (not supported
420 with the existing ULA) would be achieved by incrementing or decrementing the
421 screen start address; by one character row, it would involve adding or
422 subtracting 8. However, the ULA only supports multiples of 64 when changing the
423 screen start address. Thus, if such a scheme were to be adopted, three
424 additional bits would need to be supported in the screen start register (see
425 "Hardware Scrolling (and Enhancement)" for more details). However, horizontal
426 scrolling would be much improved even under the severe constraints of the
427 existing ULA: only adjustments of 256 to the screen start address would be
428 required to produce single-location scrolling of as few as two pixels in MODE 2
429 (four pixels in MODEs 1 and 5, eight pixels otherwise).
430
431 More disruptive is the effect of this alternative layout on software.
432 Presumably, compatibility with the BBC Micro was the primary goal of the
433 Electron's hardware design. With the character-oriented screen layout in
434 place, system software (and application software accessing the screen
435 directly) would be relying on this layout to run on the Electron with little
436 or no modification. Although it might have been possible to change the system
437 software to use this column-oriented layout instead, this would have incurred
438 a development cost and caused additional work porting things like games to the
439 Electron. Moreover, a separate branch of the software from that supporting the
440 BBC Micro and closer derivatives would then have needed maintaining.
441
442 The decision to use the character-oriented layout in the BBC Micro may have
443 been related to the choice of circuitry and to facilitate a convenient
444 hardware implementation, and by the time the Electron was planned, it was too
445 late to do anything about this somewhat unfortunate choice.
446
447 Enhancement: The Missing MODE 4
448 -------------------------------
449
450 The Electron inherits its screen mode selection from the BBC Micro, where MODE
451 3 is a text version of MODE 0, and where MODE 6 is a text version of MODE 4.
452 Neither MODE 3 nor MODE 6 is a genuine character-based text mode like MODE 7,
453 however, and they are merely implemented by skipping two scanlines in every
454 ten after the eight required to produce a character line. Thus, such modes
455 provide a 24-row display.
456
457 In principle, nothing prevents this "text mode" effect being applied to other
458 modes. The 20-column modes are not well-suited to displaying text, which
459 leaves MODE 1 which, unlike MODEs 3 and 6, can display 4 colours rather than
460 2. Although the need for a non-monochrome 40-column text mode is addressed by
461 MODE 7 on the BBC Micro, the Electron lacks such a mode.
462
463 If the 4-colour, 24-row variant of MODE 1 were to be provided, logically it
464 would occupy MODE 4 instead of the current MODE 4:
465
466 Screen mode Size (kilobytes) Colours Rows Resolution
467 ----------- ---------------- ------- ---- ----------
468 0 20 2 32 640x256
469 1 20 4 32 320x256
470 2 20 16 32 160x256
471 3 16 2 24 640x256
472 4 (new) 16 4 24 320x256
473 4 (old) 10 2 32 320x256
474 5 10 4 32 160x256
475 6 8 2 24 320x256
476
477 Thus, for increasing mode numbers, the size of each mode would be the same or
478 less than the preceding mode.
479
480 Enhancement: 2MHz RAM Access
481 ----------------------------
482
483 Given that the CPU and ULA both access RAM at 2MHz, but given that the CPU
484 when not competing with the ULA only accesses RAM every other 2MHz cycle (as
485 if the ULA still needed to access the RAM), one useful enhancement would be a
486 mechanism to let the CPU take over the ULA cycles outside the ULA's period of
487 activity comparable to the way the ULA takes over the CPU cycles in MODE 0 to
488 3.
489
490 Thus, the RAM access cycles would resemble the following in MODE 0 to 3:
491
492 Upon a transition from display cycles: UUUUCCCC (instead of UUUUC_C_)
493 On a non-display line: CCCCCCCC (instead of C_C_C_C_)
494
495 In MODE 4 to 6:
496
497 Upon a transition from display cycles: CUCUCCCC (instead of CUCUC_C_)
498 On a non-display line: CCCCCCCC (instead of C_C_C_C_)
499
500 This would improve CPU bandwidth as follows:
501
502 Standard ULA Enhanced ULA
503 MODE 0, 1, 2 9728 bytes 19456 bytes
504 MODE 3 12288 bytes 24576 bytes
505 MODE 4, 5 19968 bytes 29696 bytes
506 MODE 6 19968 bytes 32256 bytes
507
508 With such an enhancement, MODE 0 to 3 experience a doubling of CPU bandwidth
509 because all access opportunities to RAM are doubled. Meanwhile, in the other
510 modes, some CPU accesses occur alongside ULA accesses and thus cannot be
511 doubled, but the CPU bandwidth increase is still significant.
512
513 Enhancement: Region Blanking
514 ----------------------------
515
516 The problem of permitting character-oriented blitting in programs whilst
517 scrolling the screen by sub-character amounts could be mitigated by permitting
518 a region of the display to be blank, such as the final lines of the display.
519 Consider the following vertical scrolling by 2 bytes that would cause an
520 initial character row of 6 lines and a final character row of 2 lines:
521
522 6 lines - initial, partial character row
523 248 lines - 31 complete rows
524 2 lines - final, partial character row
525
526 If a routine were in use that wrote 8 line bitmaps to the partial character
527 row now split in two, it would be advisable to hide one of the regions in
528 order to prevent content appearing in the wrong place on screen (such as
529 content meant to appear at the top "leaking" onto the bottom). Blanking 6
530 lines would be sufficient, as can be seen from the following cases.
531
532 Scrolling up by 2 lines:
533
534 6 lines - initial, partial character row
535 240 lines - 30 complete rows
536 4 lines - part of 1 complete row
537 -----------------------------------------------------------------
538 4 lines - part of 1 complete row (hidden to maintain 250 lines)
539 2 lines - final, partial character row (hidden)
540
541 Scrolling down by 2 lines:
542
543 2 lines - initial, partial character row
544 248 lines - 31 complete rows
545 ----------------------------------------------------------
546 6 lines - final, partial character row (hidden)
547
548 Thus, in this case, region blanking would impose a 250 line display with the
549 bottom 6 lines blank.
550
551 See the description of the display suspend enhancement for a more efficient
552 way of blanking lines than merely blanking the palette whilst allowing the CPU
553 to perform useful work during the blanking period.
554
555 To control the blanking or suspending of lines at the top and bottom of the
556 display, a memory location could be dedicated to the task: the upper 4 bits
557 could define a blanking region of up to 16 lines at the top of the screen,
558 whereas the lower 4 bits could define such a region at the bottom of the
559 screen. If more lines were required, two locations could be employed, allowing
560 the top and bottom regions to occupy the entire screen.
561
562 Enhancement: Screen Height Adjustment
563 -------------------------------------
564
565 The height of the screen could be configurable in order to reduce screen
566 memory consumption. This is not quite done in MODE 3 and 6 since the start of
567 the screen appears to be rounded down to the nearest page, but by reducing the
568 height by amounts more than a page, savings would be possible. For example:
569
570 Screen width Depth Height Bytes per line Saving in bytes Start address
571 ------------ ----- ------ -------------- --------------- -------------
572 640 1 252 80 320 &3140 -> &3100
573 640 1 248 80 640 &3280 -> &3200
574 320 1 240 40 640 &5A80 -> &5A00
575 320 2 240 80 1280 &3500
576
577 Screen Mode Selection
578 ---------------------
579
580 Bits 3, 4 and 5 of address &FE*7 control the selected screen mode. For a wider
581 range of modes, the other bits of &FE*7 (related to sound, cassette
582 input/output and the Caps Lock LED) would need to be reassigned and bit 0
583 potentially being made available for use.
584
585 Enhancement: Palette Definition
586 -------------------------------
587
588 Since all memory accesses go via the ULA, an enhanced ULA could employ more
589 specific addresses than &FE*X to perform enhanced functions. For example, the
590 palette control is done using &FE*8-F and merely involves selecting predefined
591 colours, whereas an enhanced ULA could support the redefinition of all 16
592 colours using specific ranges such as &FE18-F (colours 0 to 7) and &FE28-F
593 (colours 8 to 15), where a single byte might provide 8 bits per pixel colour
594 specifications similar to those used on the Archimedes.
595
596 The principal limitation here is actually the hardware: the Electron has only
597 a single output line for each of the red, green and blue channels, and if
598 those outputs are strictly digital and can only be set to a "high" and "low"
599 value, then only the existing eight colours are possible. If a modern ULA were
600 able to output analogue values (or values at well-defined points between the
601 high and low values, such as the half-on value supported by the Amstrad CPC
602 series), it would still need to be assessed whether the circuitry could
603 successfully handle and propagate such values. Various sources indicate that
604 only "TTL levels" are supported by the RGB output circuit, and since there are
605 74LS08 AND logic gates involved in the RGB component outputs from the ULA, it
606 is likely that the ULA is expected to provide only "high" or "low" values.
607
608 Short of adding extra outputs from the ULA (either additional red, green and
609 blue outputs or a combined intensity output), another approach might involve
610 some kind of modulation where an output value might be encoded in multiple
611 pulses at a higher frequency than the pixel frequency. However, this would
612 demand additional circuitry outside the ULA, and component RGB monitors would
613 probably not be able to take advantage of this feature; only UHF and composite
614 video devices (the latter with the composite video colour support enabled on
615 the Electron's circuit board) would potentially benefit.
616
617 Flashing Colours
618 ----------------
619
620 According to the Advanced User Guide, "The cursor and flashing colours are
621 entirely generated in software: This means that all of the logical to physical
622 colour map must be changed to cause colours to flash." This appears to suggest
623 that the palette registers must be updated upon the flash counter - read and
624 written by OSBYTE &C1 (193) - reaching zero and that some way of changing the
625 colour pairs to be any combination of colours might be possible, instead of
626 having colour complements as pairs.
627
628 It is conceivable that the interrupt code responsible does the simple thing
629 and merely inverts the current values for any logical colours (LC) for which
630 the associated physical colour (as supplied as the second parameter to the VDU
631 19 call) has the top bit of its four bit value set. These top bits are not
632 recorded in the palette registers but are presumably recorded separately and
633 used to build bitmaps as follows:
634
635 LC 2 colour 4 colour 16 colour 4-bit value for inversion
636 -- -------- -------- --------- -------------------------
637 0 00010001 00010001 00010001 1, 1, 1
638 1 01000100 00100010 00010001 4, 2, 1
639 2 01000100 00100010 4, 2
640 3 10001000 00100010 8, 2
641 4 00010001 1
642 5 00010001 1
643 6 00100010 2
644 7 00100010 2
645 8 01000100 4
646 9 01000100 4
647 10 10001000 8
648 11 10001000 8
649 12 01000100 4
650 13 01000100 4
651 14 10001000 8
652 15 10001000 8
653
654 Inversion value calculation:
655
656 2 colour formula: 1 << (colour * 2)
657 4 colour formula: 1 << colour
658 16 colour formula: 1 << ((colour & 2) + ((colour & 8) * 2))
659
660 For example, where logical colour 0 has been mapped to a physical colour in
661 the range 8 to 15, a bitmap of 00010001 would be chosen as its contribution to
662 the inversion operation. (The lower three bits of the physical colour would be
663 used to set the underlying colour information affected by the inversion
664 operation.)
665
666 An operation in the interrupt code would then combine the bitmaps for all
667 logical colours in 2 and 4 colour modes, with the 16 colour bitmaps being
668 combined for groups of logical colours as follows:
669
670 Logical colours
671 ---------------
672 0, 2, 8, 10
673 4, 6, 12, 14
674 5, 7, 13, 15
675 1, 3, 9, 11
676
677 These combined bitmaps would be EORed with the existing palette register
678 values in order to perform the value inversion necessary to produce the
679 flashing effect.
680
681 Thus, in the VDU 19 operation, the appropriate inversion value would be
682 calculated for the logical colour, and this value would then be combined with
683 other inversion values in a dedicated memory location corresponding to the
684 colour's group as indicated above. Meanwhile, the palette channel values would
685 be derived from the lower three bits of the specified physical colour and
686 combined with other palette data in dedicated memory locations corresponding
687 to the palette registers.
688
689 Interestingly, although flashing colours on the BBC Micro are controlled by
690 toggling bit 0 of the &FE20 control register location for the Video ULA, the
691 actual colour inversion is done in hardware.
692
693 Enhancement: Palette Definition Lists
694 -------------------------------------
695
696 It can be useful to redefine the palette in order to change the colours
697 available for a particular region of the screen, particularly in modes where
698 the choice of colours is constrained, and if an increased colour depth were
699 available, palette redefinition would be useful to give the illusion of more
700 than 16 colours in MODE 2. Traditionally, palette redefinition has been done
701 by using interrupt-driven timers, but a more efficient approach would involve
702 presenting lists of palette definitions to the ULA so that it can change the
703 palette at a particular display line.
704
705 One might define a palette redefinition list in a region of memory and then
706 communicate its contents to the ULA by writing the address and length of the
707 list, along with the display line at which the palette is to be changed, to
708 ULA registers such that the ULA buffers the list and performs the redefinition
709 at the appropriate time. Throughput/bandwidth considerations might impose
710 restrictions on the practical length of such a list, however.
711
712 Enhancement: Display Synchronisation Interrupts
713 -----------------------------------------------
714
715 When completing each scanline of the display, the ULA could trigger an
716 interrupt. Since this might impact system performance substantially, the
717 feature would probably need to be configurable, and it might be sufficient to
718 have an interrupt only after a certain number of display lines instead.
719 Permitting the CPU to take action after eight lines would allow palette
720 switching and other effects to occur on a character row basis.
721
722 The ULA provides an interrupt at the end of the display period, presumably so
723 that software can schedule updates to the screen, avoid flickering or tearing,
724 and so on. However, some applications might benefit from an interrupt at, or
725 just before, the start of the display period so that palette modifications or
726 similar effects could be scheduled.
727
728 Enhancement: Palette-Free Modes
729 -------------------------------
730
731 Palette-free modes might be defined where bit values directly correspond to
732 the red, green and blue channels, although this would mostly make sense only
733 for modes with depths greater than the standard 4 bits per pixel, and such
734 modes would require more memory than MODE 2 if they were to have an acceptable
735 resolution.
736
737 Enhancement: Display Suspend
738 ----------------------------
739
740 Especially when writing to the screen memory, it could be beneficial to be
741 able to suspend the ULA's access to the memory, instead producing blank values
742 for all screen pixels until a program is ready to reveal the screen. This is
743 different from palette blanking since with a blank palette, the ULA is still
744 reading screen memory and translating its contents into pixel values that end
745 up being blank.
746
747 This function is reminiscent of a capability of the ZX81, albeit necessary on
748 that hardware to reduce the load on the system CPU which was responsible for
749 producing the video output. By allowing display suspend on the Electron, the
750 performance benefit would be derived from giving the CPU full access to the
751 memory bandwidth.
752
753 The region blanking feature mentioned above could be implemented using this
754 enhancement instead of employing palette blanking for the affected lines of
755 the display.
756
757 Enhancement: Memory Filling
758 ---------------------------
759
760 A capability that could be given to an enhanced ULA is that of permitting the
761 ULA to write to screen memory as well being able to read from it. Although
762 such a capability would probably not be useful in conjunction with the
763 existing read operations when producing a screen display, and insufficient
764 bandwidth would exist to do so in high-bandwidth screen modes anyway, the
765 capability could be offered during a display suspend period (as described
766 above), permitting a more efficient mechanism to rapidly fill memory with a
767 predetermined value.
768
769 This capability could also support block filling, where the limits of the
770 filled memory would be defined by the position and size of a screen area,
771 although this would demand the provision of additional registers in the ULA to
772 retain the details of such areas and additional logic to control the fill
773 operation.
774
775 Enhancement: Region Filling
776 ---------------------------
777
778 An alternative to memory writing might involve indicating regions using
779 additional registers or memory where the ULA fills regions of the screen with
780 content instead of reading from memory. Unlike hardware sprites which should
781 realistically provide varied content, region filling could employ single
782 colours or patterns, and one advantage of doing so would be that the ULA need
783 not access memory at all within a particular region.
784
785 Regions would be defined on a row-by-row basis. Instead of reading memory and
786 blitting a direct representation to the screen, the ULA would read region
787 definitions containing a start column, region width and colour details. There
788 might be a certain number of definitions allowed per row, or the ULA might
789 just traverse an ordered list of such definitions with each one indicating the
790 row, start column, region width and colour details.
791
792 One could even compress this information further by requiring only the row,
793 start column and colour details with each subsequent definition terminating
794 the effect of the previous one. However, one would also need to consider the
795 convenience of preparing such definitions and whether efficient access to
796 definitions for a particular row might be desirable. It might also be
797 desirable to avoid having to prepare definitions for "empty" areas of the
798 screen, effectively making the definition of the screen contents employ
799 run-length encoding and employ only colour plus length information.
800
801 One application of region filling is that of simple 2D and 3D shape rendering.
802 Although it is entirely possible to plot such shapes to the screen and have
803 the ULA blit the memory contents to the screen, such operations consume
804 bandwidth both in the initial plotting and in the final transfer to the
805 screen. Region filling would reduce such bandwidth usage substantially.
806
807 This way of representing screen images would make certain kinds of images
808 unfeasible to represent - consider alternating single pixel values which could
809 easily occur in some character bitmaps - even if an internal queue of regions
810 were to be supported such that the ULA could read ahead and buffer such
811 "bandwidth intensive" areas. Thus, the ULA might be better served providing
812 this feature for certain areas of the display only as some kind of special
813 graphics window.
814
815 Enhancement: Hardware Sprites
816 -----------------------------
817
818 An enhanced ULA might provide hardware sprites, but this would be done in an
819 way that is incompatible with the standard ULA, since no &FE*X locations are
820 available for allocation. To keep the facility simple, hardware sprites would
821 have a standard byte width and height.
822
823 The specification of sprites could involve the reservation of 16 locations
824 (for example, &FE20-F) specifying a fixed number of eight sprites, with each
825 location pair referring to the sprite data. By limiting the ULA to dealing
826 with a fixed number of sprites, the work required inside the ULA would be
827 reduced since it would avoid having to deal with arbitrary numbers of sprites.
828
829 The principal limitation on providing hardware sprites is that of having to
830 obtain sprite data, given that the ULA is usually required to retrieve screen
831 data, and given the lack of memory bandwidth available to retrieve sprite data
832 (particularly from multiple sprites supposedly at the same position) and
833 screen data simultaneously. Although the ULA could potentially read sprite
834 data and screen data in alternate memory accesses in screen modes where the
835 bandwidth is not already fully utilised, this would result in a degradation of
836 performance.
837
838 Enhancement: Additional Screen Mode Configurations
839 --------------------------------------------------
840
841 Alternative screen mode configurations could be supported. The ULA has to
842 produce 640 pixel values across the screen, with pixel doubling or quadrupling
843 employed to fill the screen width:
844
845 Screen width Columns Scaling Depth Bytes
846 ------------ ------- ------- ----- -----
847 640 80 x1 1 80
848 320 40 x2 1, 2 40, 80
849 160 20 x4 2, 4 40, 80
850
851 It must also use at most 80 byte-sized memory accesses to provide the
852 information for the display. Given that characters must occupy an 8x8 pixel
853 array, if a configuration featuring anything other than 20, 40 or 80 character
854 columns is to be supported, compromises must be made such as the introduction
855 of blank pixels either between characters (such as occurs between rows in MODE
856 3 and 6) or at the end of a scanline (such as occurs at the end of the frame
857 in MODE 3 and 6). Consider the following configuration:
858
859 Screen width Columns Scaling Depth Bytes Blank
860 ------------ ------- ------- ----- ------ -----
861 208 26 x3 1, 2 26, 52 16
862
863 Here, if the ULA can triple pixels, a 26 column mode with either 2 or 4
864 colours could be provided, with 16 blank pixel values (out of a total of 640)
865 generated either at the start or end (or split between the start and end) of
866 each scanline.
867
868 Enhancement: Character Attributes
869 ---------------------------------
870
871 The BBC Micro MODE 7 employs something resembling character attributes to
872 support teletext displays, but depends on circuitry providing a character
873 generator. The ZX Spectrum, on the other hand, provides character attributes
874 as a means of colouring bitmapped graphics. Although such a feature is very
875 limiting as the sole means of providing multicolour graphics, in situations
876 where the choice is between low resolution multicolour graphics or high
877 resolution monochrome graphics, character attributes provide a potentially
878 useful compromise.
879
880 For each byte read, the ULA must deliver 8 pixel values (out of a total of
881 640) to the video output, doing so by either emptying its pixel buffer on a
882 pixel per cycle basis, or by multiplying pixels and thus holding them for more
883 than one cycle. For example for a screen mode having 640 pixels in width:
884
885 Cycle: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
886 Reads: B B
887 Pixels: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
888
889 And for a screen mode having 320 pixels in width:
890
891 Cycle: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
892 Reads: B
893 Pixels: 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7
894
895 However, in modes where less than 80 bytes are required to generate the pixel
896 values, an enhanced ULA might be able to read additional bytes between those
897 providing the bitmapped graphics data:
898
899 Cycle: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
900 Reads: B A
901 Pixels: 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7
902
903 These additional bytes could provide colour information for the bitmapped data
904 in the following character column (of 8 pixels). Since it would be desirable
905 to apply attribute data to the first column, the initial 8 cycles might be
906 configured to not produce pixel values.
907
908 For an entire character, attribute data need only be read for the first row of
909 pixels for a character. The subsequent rows would have attribute information
910 applied to them, although this would require the attribute data to be stored
911 in some kind of buffer. Thus, the following access pattern would be observed:
912
913 Cycle: A B ... _ B ... _ B ... _ B ... _ B ... _ B ... _ B ... _ B ...
914
915 A whole byte used for colour information for a whole character would result in
916 a choice of 256 colours, and this might be somewhat excessive. By only reading
917 attribute bytes at every other opportunity, a choice of 16 colours could be
918 applied individually to two characters.
919
920 Cycle: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
921 Reads: B A B -
922 Pixels: 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7
923
924 Further reductions in attribute data access, offering 4 colours for every
925 character in a four character block, for example, might also be worth
926 considering.
927
928 Consider the following configurations for screen modes with a colour depth of
929 1 bit per pixel for bitmap information:
930
931 Screen width Columns Scaling Bytes (B) Bytes (A) Colours Screen start
932 ------------ ------- ------- --------- --------- ------- ------------
933 320 40 x2 40 40 256 &5300
934 320 40 x2 40 20 16 &5580 -> &5500
935 320 40 x2 40 10 4 &56C0 -> &5600
936 208 26 x3 26 26 256 &62C0 -> &6200
937 208 26 x3 26 13 16 &6460 -> &6400
938
939 Enhancement: MODE 7 Emulation using Character Attributes
940 --------------------------------------------------------
941
942 If the scheme of applying attributes to character regions were employed to
943 emulate MODE 7, in conjunction with the MODE 6 display technique, the
944 following configuration would be required:
945
946 Screen width Columns Rows Bytes (B) Bytes (A) Colours Screen start
947 ------------ ------- ---- --------- --------- ------- ------------
948 320 40 25 40 20 16 &5ECC -> &5E00
949 320 40 25 40 10 4 &5FC6 -> &5F00
950
951 Although this requires much more memory than MODE 7 (8500 bytes versus MODE
952 7's 1000 bytes), it does not need much more memory than MODE 6, and it would
953 at least make a limited 40-column multicolour mode available as a substitute
954 for MODE 7.
955
956 Enhancement: High Resolution Graphics
957 -------------------------------------
958
959 Screen modes with higher resolutions and larger colour depths might be
960 possible, but this would in most cases involve the allocation of more screen
961 memory, and the ULA would probably then be obliged to page in such memory for
962 the CPU to be able to sensibly access it all.
963
964 Enhancement: Genlock Support
965 ----------------------------
966
967 The ULA generates a video signal in conjunction with circuitry producing the
968 output features necessary for the correct display of the screen image.
969 However, it appears that the ULA drives the video synchronisation mechanism
970 instead of reacting to an existing signal. Genlock support might be possible
971 if the ULA were made to be responsive to such external signals, resetting its
972 address generators upon receiving synchronisation events.
973
974 Enhancement: Improved Sound
975 ---------------------------
976
977 The standard ULA reserves &FE*6 for sound generation and cassette input/output
978 (with bits 1 and 2 of &FE*7 being used to select either sound generation or
979 cassette I/O), thus making it impossible to support multiple channels within
980 the given framework. The BBC Micro ULA employs &FE40-&FE4F for sound control,
981 and an enhanced ULA could adopt this interface.
982
983 The BBC Micro uses the SN76489 chip to produce sound, and the entire
984 functionality of this chip could be emulated for enhanced sound, with a subset
985 of the functionality exposed via the &FE*6 interface.
986
987 See: http://en.wikipedia.org/wiki/Texas_Instruments_SN76489
988 See: http://www.smspower.org/Development/SN76489
989
990 Enhancement: Waveform Upload
991 ----------------------------
992
993 As with a hardware sprite function, waveforms could be uploaded or referenced
994 using locations as registers referencing memory regions.
995
996 Enhancement: Sound Input/Output
997 -------------------------------
998
999 Since the ULA already controls audio input/output for cassette-based data, it
1000 would have been interesting to entertain the idea of sampling and output of
1001 sounds through the cassette interface. However, a significant amount of
1002 circuitry is employed to process the input signal for use by the ULA and to
1003 process the output signal for recording.
1004
1005 See: http://bbc.nvg.org/doc/A%20Hardware%20Guide%20for%20the%20BBC%20Microcomputer/bbc_hw_03.htm#3.11
1006
1007 Enhancement: BBC ULA Compatibility
1008 ----------------------------------
1009
1010 Although some new ULA functions could be defined in a way that is also
1011 compatible with the BBC Micro, the BBC ULA is itself incompatible with the
1012 Electron ULA: &FE00-7 is reserved for the video controller in the BBC memory
1013 map, but controls various functions specific to the 6845 video controller;
1014 &FE08-F is reserved for the serial controller. It therefore becomes possible
1015 to disregard compatibility where compatibility is already disregarded for a
1016 particular area of functionality.
1017
1018 &FE20-F maps to video ULA functionality on the BBC Micro which provides
1019 control over the palette (using address &FE21, compared to &FE07-F on the
1020 Electron) and other system-specific functions. Since the location usage is
1021 generally incompatible, this region could be reused for other purposes.
1022
1023 Enhancement: Increased RAM, ULA and CPU Performance
1024 ---------------------------------------------------
1025
1026 More modern implementations of the hardware might feature faster RAM coupled
1027 with an increased ULA clock frequency in order to increase the bandwidth
1028 available to the ULA and to the CPU in situations where the ULA is not needed
1029 to perform work. A ULA employing a 32MHz clock would be able to complete the
1030 retrieval of a byte from RAM in only 250ns and thus be able to enable the CPU
1031 to access the RAM for the following 250ns even in display modes requiring the
1032 retrieval of a byte for the display every 500ns. The CPU could, subject to
1033 timing issues, run at 2MHz even in MODE 0, 1 and 2.
1034
1035 A scheme such as that described above would have a similar effect to the
1036 scheme employed in the BBC Micro, although the latter made use of RAM with a
1037 wider bandwidth in order to complete memory transfers within 250ns and thus
1038 permit the CPU to run continuously at 2MHz.
1039
1040 Higher bandwidth could potentially be used to implement exotic features such
1041 as RAM-resident hardware sprites or indeed any feature demanding RAM access
1042 concurrent with the production of the display image.
1043
1044 Enhancement: Multiple CPU Stacks and Zero Pages
1045 -----------------------------------------------
1046
1047 The 6502 maintains a stack for subroutine calls and register storage in page
1048 &01. Although the stack register can be manipulated using the TSX and TXS
1049 instructions, thereby permitting the maintenance of multiple stack regions and
1050 thus the potential coexistence of multiple programs each using a separate
1051 region, only programs that make little use of the stack (perhaps avoiding
1052 deeply-nested subroutine invocations and significant register storage) would
1053 be able to coexist without overwriting each other's stacks.
1054
1055 One way that this issue could be alleviated would involve the provision of a
1056 facility to redirect accesses to page &01 to other areas of memory. The ULA
1057 would provide a register that defines a physical page for the use of the CPU's
1058 "logical" page &01, and upon any access to page &01 by the CPU, the ULA would
1059 change the asserted address lines to redirect the access to the appropriate
1060 physical region.
1061
1062 By providing an 8-bit register, mapping to the most significant byte (MSB) of
1063 a 16-bit address, the ULA could then replace any MSB equal to &01 with the
1064 register value before the access is made. Where multiple programs coexist,
1065 upon switching programs, the register would be updated to point the ULA to the
1066 appropriate stack location, thus providing a simple memory management unit
1067 (MMU) capability.
1068
1069 In a similar fashion, zero page accesses could also be redirected so that code
1070 could run from sideways RAM and have zero page operations redirected to "upper
1071 memory" - for example, to page &BE (with stack accesses redirected to page
1072 &BF, perhaps) - thereby permitting most CPU operations to occur without
1073 inadvertent accesses to "lower memory" (the RAM) which would risk stalling the
1074 CPU as it contends with the ULA for memory access.
1075
1076 Such facilities could also be provided by a separate circuit between the CPU
1077 and ULA in a fashion similar to that employed by a "turbo" board, but unlike
1078 such boards, no additional RAM would be provided: all memory accesses would
1079 occur as normal through the ULA, albeit redirected when configured
1080 appropriately.
1081
1082 ULA Pin Functions
1083 -----------------
1084
1085 The functions of the ULA pins are described in the Electron Service Manual. Of
1086 interest to video processing are the following:
1087
1088 CSYNC (low during horizontal or vertical synchronisation periods, high
1089 otherwise)
1090
1091 HS (low during horizontal synchronisation periods, high otherwise)
1092
1093 RED, GREEN, BLUE (pixel colour outputs)
1094
1095 CLOCK IN (a 16MHz clock input, 4V peak to peak)
1096
1097 PHI OUT (a 1MHz, 2MHz and stopped clock signal for the CPU)
1098
1099 More general memory access pins:
1100
1101 RAM0...RAM3 (data lines to/from the RAM)
1102
1103 RA0...RA7 (address lines for sending both row and column addresses to the RAM)
1104
1105 RAS (row address strobe setting the row address on a negative edge - see the
1106 timing notes)
1107
1108 CAS (column address strobe setting the column address on a negative edge -
1109 see the timing notes)
1110
1111 WE (sets write enable with logic 0, read with logic 1)
1112
1113 ROM (select data access from ROM)
1114
1115 CPU-oriented memory access pins:
1116
1117 A0...A15 (CPU address lines)
1118
1119 PD0...PD7 (CPU data lines)
1120
1121 R/W (indicates CPU write with logic 0, CPU read with logic 1)
1122
1123 Interrupt-related pins:
1124
1125 NMI (CPU request for uninterrupted 1MHz access to memory)
1126
1127 IRQ (signal event to CPU)
1128
1129 POR (power-on reset, resetting the ULA on a positive edge and asserting the
1130 CPU's RST pin)
1131
1132 RST (master reset for the CPU signalled on power-up and by the Break key)
1133
1134 Keyboard-related pins:
1135
1136 KBD0...KBD3 (keyboard inputs)
1137
1138 CAPS LOCK (control status LED)
1139
1140 Sound-related pins:
1141
1142 SOUND O/P (sound output using internal oscillator)
1143
1144 Cassette-related pins:
1145
1146 CAS IN (cassette circuit input, between 0.5V to 2V peak to peak)
1147
1148 CAS OUT (pseudo-sinusoidal output, 1.8V peak to peak)
1149
1150 CAS RC (detect high tone)
1151
1152 CAS MO (motor relay output)
1153
1154 ÷13 IN (~1200 baud clock input)
1155
1156 ULA Socket
1157 ----------
1158
1159 The socket used for the ULA is a 3M/TexTool 268-5400 68-pin socket.
1160
1161 References
1162 ----------
1163
1164 See: http://bbc.nvg.org/doc/A%20Hardware%20Guide%20for%20the%20BBC%20Microcomputer/bbc_hw.htm
1165
1166 About this Document
1167 -------------------
1168
1169 The most recent version of this document and accompanying distribution should
1170 be available from the following location:
1171
1172 http://hgweb.boddie.org.uk/ULA
1173
1174 Copyright and licence information can be found in the docs directory of this
1175 distribution - see docs/COPYING.txt for more information.