1 Timing
2 ------
3
4 According to the above (15.3.2 in the AUG), there are 312 scanlines, 256 of
5 which are used to generate pixel data. At 50Hz, this means that 128 cycles are
6 used to produce pixel data (2000000 / 50 = 40000; 40000 / 312 ~= 128). This is
7 consistent with the observation that each scanline requires at most 80 bytes
8 of data, and that the ULA is apparently busy for 40 out of 64 microseconds in
9 each scanline.
10
11 See: The Advanced User Guide for the Acorn Electron
12 See: http://mdfs.net/Docs/Comp/Electron/Techinfo.htm
13
14 Hardware Scrolling
15 ------------------
16
17 On the standard ULA, &FE02 and &FE03 map to a 9 significant bits address with
18 the least significant 5 bits being zero, thus limiting the scrolling
19 resolution to 64 bytes. An enhanced ULA could support a resolution of 2 bytes
20 using the same layout of these addresses.
21
22 |--&FE02--------------| |--&FE03--------------|
23 XX XX 14 13 12 11 10 09 08 07 06 XX XX XX XX XX
24
25 XX 14 13 12 11 10 09 08 07 06 05 04 03 02 01 XX
26
27 Arguably, a resolution of 8 bytes is more useful, since the mapping of screen
28 memory to pixel locations is character oriented. A change in 8 bytes would
29 permit a horizontal scrolling resolution of 2 pixels in MODE 2, 4 pixels in
30 MODE 1 and 5, and 8 pixels in MODE 0, 3 and 6. This resolution is actually
31 observed on the BBC Micro (see 18.11.2 in the BBC Microcomputer Advanced User
32 Guide).
33
34 One argument for a 2 byte resolution is smooth vertical scrolling. A pitfall
35 of changing the screen address by 2 bytes is the change in the number of lines
36 from the initial and final character rows that need reading by the ULA, which
37 would need to maintain this state information (although this is a relatively
38 trivial change). Another pitfall is the complication that might be introduced
39 to software writing bitmaps of character height to the screen.
40
41 Region Blanking
42 ---------------
43
44 The problem of permitting character-oriented blitting in programs whilst
45 scrolling the screen by sub-character amounts could be mitigated by permitting
46 a region of the display to be blank, such as the final lines of the display.
47 Consider the following vertical scrolling by 2 bytes that would cause an
48 initial character row of 6 lines and a final character row of 2 lines:
49
50 6 lines - initial, partial character row
51 248 lines - 31 complete rows
52 2 lines - final, partial character row
53
54 If a routine were in use that wrote 8 line bitmaps to the partial character
55 row now split in two, it would be advisable to hide one of the regions in
56 order to prevent content appearing in the wrong place on screen (such as
57 content meant to appear at the top "leaking" onto the bottom). Blanking 6
58 lines would be sufficient, as can be seen from the following cases.
59
60 Scrolling up by 2 lines:
61
62 6 lines - initial, partial character row
63 240 lines - 30 complete rows
64 4 lines - part of 1 complete row
65 -----------------------------------------------------------------
66 4 lines - part of 1 complete row (hidden to maintain 250 lines)
67 2 lines - final, partial character row (hidden)
68
69 Scrolling down by 2 lines:
70
71 2 lines - initial, partial character row
72 248 lines - 31 complete rows
73 ----------------------------------------------------------
74 6 lines - final, partial character row (hidden)
75
76 Thus, region blanking would impose a 250 line display with the bottom 6 lines
77 blank. The height of the screen could be configurable still further.
78
79 Palette Definition
80 ------------------
81
82 Since all memory accesses go via the ULA, an enhanced ULA could employ more
83 specific addresses than &FE*X to perform enhanced functions. For example, the
84 palette control is done using &FE*8-F and merely involves selecting predefined
85 colours, whereas an enhanced ULA could support the redefinition of all 16
86 colours using specific ranges such as &FE18-F (colours 0 to 7) and &FE28-F
87 (colours 8 to 15), where a single byte might provide 8 bits per pixel colour
88 specifications similar to those used on the Archimedes.
89
90 The principal limitation here is actually the hardware: the Electron has only
91 a single output line for each of the red, green and blue channels, and if
92 those outputs are strictly digital and can only be set to a "high" and "low"
93 value, then only the existing eight colours are possible. If a modern ULA were
94 able to output analogue values, it would still need to be assessed whether the
95 circuitry could successfully handle and propagate such values.
96
97 Palette Definition Lists
98 ------------------------
99
100 It can be useful to redefine the palette in order to change the colours
101 available for a particular region of the screen, particularly in modes where
102 the choice of colours is constrained, and if an increased colour depth were
103 available, palette redefinition would be useful to give the illusion of more
104 than 16 colours in MODE 2. Traditionally, palette redefinition has been done
105 by using interrupt-driven timers, but a more efficient approach would involve
106 presenting lists of palette definitions to the ULA so that it can change the
107 palette at a particular display line.
108
109 One might define a palette redefinition list in a region of memory and then
110 communicate its contents to the ULA by writing the address and length of the
111 list, along with the display line at which the palette is to be changed, to
112 ULA registers such that the ULA buffers the list and performs the redefinition
113 at the appropriate time. Throughput/bandwidth considerations might impose
114 restrictions on the practical length of such a list, however.
115
116 Palette-Free Modes
117 ------------------
118
119 Palette-free modes might be defined where bit values directly correspond to
120 the red, green and blue channels, although this would mostly make sense only
121 for modes with depths greater than the standard 4 bits per pixel, and such
122 modes would require more memory than MODE 2 if they were to have an acceptable
123 resolution.
124
125 Display Suspend
126 ---------------
127
128 Especially when writing to the screen memory, it could be beneficial to be
129 able to suspend the ULA's access to the memory, instead producing blank values
130 for all screen pixels until a program is ready to reveal the screen. This is
131 different from palette blanking since with a blank palette, the ULA is still
132 reading screen memory and translating its contents into pixel values that end
133 up being blank.
134
135 This function is reminiscent of a capability of the ZX81, albeit necessary on
136 that hardware to reduce the load on the system CPU which was responsible for
137 producing the video output.
138
139 Hardware Sprites and Colour Planes
140 ----------------------------------
141
142 An enhanced ULA might provide hardware sprites, but this would be done in an
143 way that is incompatible with the standard ULA, since no &FE*X locations are
144 available for allocation. In a special ULA mode, one might allocate a pair of
145 locations (for example, &FE20 and &FE21) as a pair of registers referencing a
146 region of memory from which a sprite might be found and potentially copied
147 into internal RAM, with other locations (for example, &FE22 and &FE23)
148 providing the size of the region. Alternatively, one might write the region
149 location and size through a single ULA location, with the ULA being put into a
150 particular state after each write. For example: read LSB of region, read MSB
151 of region, read size, read height.
152
153 Providing hardware sprites can be awkward without having some kind of working
154 area, since the ULA would need to remember where each sprite is to be plotted
155 and then deduce which sprites would be contributing to any given pixel. An
156 alternative is to use memory into which the sprites would be plotted, and this
157 memory would be combined with the main screen memory, taking a particular
158 colour as the "colourkey" which is to be considered transparent, and only
159 overwriting the main screen pixels with pixel values for other colours.
160
161 Enhanced Graphics
162 -----------------
163
164 Screen modes with different screen memory mappings, higher resolutions and
165 larger colour depths might be possible, but this would in most cases involve
166 the allocation of more screen memory, and the ULA would probably then be
167 obliged to page in such memory for the CPU to be able to sensibly access it
168 all. Merely changing the memory mappings in order to have Archimedes-style
169 row-oriented screen addresses (instead of character-oriented addresses) could
170 be done for the existing modes, but this might not be sufficiently beneficial,
171 especially since accessing regions of the screen would involve incrementing
172 pointers by amounts that are inconvenient on an 8-bit CPU.
173
174 Enhanced Sound
175 --------------
176
177 The standard ULA reserves &FE*6 for sound generation and cassette
178 input/output, thus making it impossible to support multiple channels within
179 the given framework. The BBC Micro ULA employs &FE40-&FE4F for sound control,
180 and an enhanced ULA could adopt this interface.
181
182 The BBC Micro uses the SN76489 chip to produce sound, and the entire
183 functionality of this chip could be emulated for enhanced sound, with a subset
184 of the functionality exposed via the &FE*6 interface.
185
186 See: http://en.wikipedia.org/wiki/Texas_Instruments_SN76489
187
188 Waveform Upload
189 ---------------
190
191 As with a hardware sprite function, waveforms could be uploaded or referenced
192 using locations as registers referencing memory regions.
193
194 BBC ULA Compatibility
195 ---------------------
196
197 Although some new ULA functions could be defined in a way that is also
198 compatible with the BBC Micro, the BBC ULA is itself incompatible with the
199 Electron ULA: &FE00-7 is reserved for the video controller in the BBC memory
200 map, but controls various functions specific to the 6845 video controller;
201 &FE08-F is reserved for the serial controller. It therefore becomes possible
202 to disregard compatibility where compatibility is already disregarded for a
203 particular area of functionality.
204
205 &FE20-F maps to video ULA functionality on the BBC Micro which provides
206 control over the palette (using address &FE21, compared to &FE07-F on the
207 Electron) and other system-specific functions. Since the location usage is
208 generally incompatible, this region could be reused for other purposes.