1 Hardware Scrolling
2 ------------------
3
4 On the standard ULA, &FE02 and &FE03 map to a 9 significant bits address with
5 the least significant 5 bits being zero, thus limiting the scrolling
6 resolution to 64 bytes. An enhanced ULA could support a resolution of 2 bytes
7 using the same layout of these addresses.
8
9 |--&FE02--------------| |--&FE03--------------|
10 XX XX 14 13 12 11 10 09 08 07 06 XX XX XX XX XX
11
12 XX 14 13 12 11 10 09 08 07 06 05 04 03 02 01 XX
13
14 Arguably, a resolution of 8 bytes is more useful, since the mapping of screen
15 memory to pixel locations is character oriented. A change in 8 bytes would
16 permit a horizontal scrolling resolution of 2 pixels in MODE 2, 4 pixels in
17 MODE 1 and 5, and 8 pixels in MODE 0, 3 and 6. This resolution is actually
18 observed on the BBC Micro (see 18.11.2 in the BBC Microcomputer Advanced User
19 Guide).
20
21 One argument for a 2 byte resolution is smooth vertical scrolling. A pitfall
22 of changing the screen address by 2 bytes is the change in the number of lines
23 from the initial and final character rows that need reading by the ULA, which
24 would need to maintain this state information. Another pitfall is the
25 complication that might be introduced to software writing bitmaps of character
26 height to the screen.
27
28 Region Blanking
29 ---------------
30
31 The problem of permitting character-oriented blitting in programs whilst
32 scrolling the screen by sub-character amounts could be mitigated by permitting
33 a region of the display to be blank, such as the final lines of the display.
34 Consider the following vertical scrolling by 2 bytes that would cause an
35 initial character row of 6 lines and a final character row of 2 lines:
36
37 6 lines - initial, partial character row
38 248 lines - 31 complete rows
39 2 lines - final, partial character row
40
41 If a routine were in use that wrote 8 line bitmaps to the partial character
42 row now split in two, it would be advisable to hide one of the regions in
43 order to prevent content appearing in the wrong place on screen (such as
44 content meant to appear at the top "leaking" onto the bottom). Blanking 6
45 lines would be sufficient, as can be seen from the following cases.
46
47 Scrolling up by 2 lines:
48
49 6 lines - initial, partial character row
50 240 lines - 30 complete rows
51 4 lines - part of 1 complete row
52 -----------------------------------------------------------------
53 4 lines - part of 1 complete row (hidden to maintain 250 lines)
54 2 lines - final, partial character row (hidden)
55
56 Scrolling down by 2 lines:
57
58 2 lines - initial, partial character row
59 248 lines - 31 complete rows
60 ----------------------------------------------------------
61 6 lines - final, partial character row (hidden)
62
63 Thus, region blanking would impose a 250 line display with the bottom 6 lines
64 blank. The height of the screen could be configurable still further.
65
66 Palette Definition
67 ------------------
68
69 Since all memory accesses go via the ULA, an enhanced ULA could employ more
70 specific addresses than &FE*X to perform enhanced functions. For example, the
71 palette control is done using &FE*8-F and merely involves selecting predefined
72 colours, whereas an enhanced ULA could support the redefinition of all 16
73 colours using specific ranges such as &FE18-F (colours 0 to 7) and &FE28-F
74 (colours 8 to 15), where a single byte might provide 8 bits per pixel colour
75 specifications similar to those used on the Archimedes.
76
77 The principal limitation here is actually the hardware: the Electron has only
78 a single output line for each of the red, green and blue channels, and if
79 those outputs are strictly digital and can only be set to a "high" and "low"
80 value, then only the existing eight colours are possible. If a modern ULA were
81 able to output analogue values, it would still need to be assessed whether the
82 circuitry could successfully handle and propagate such values.
83
84 Palette Definition Lists
85 ------------------------
86
87 It can be useful to redefine the palette in order to change the colours
88 available for a particular region of the screen, particularly in modes where
89 the choice of colours is constrained, and if an increased colour depth were
90 available, palette redefinition would be useful to give the illusion of more
91 than 16 colours in MODE 2. Traditionally, palette redefinition has been done
92 by using interrupt-driven timers, but a more efficient approach would involve
93 presenting lists of palette definitions to the ULA so that it can change the
94 palette at a particular display line.
95
96 One might define a palette redefinition list in a region of memory and then
97 communicate its contents to the ULA by writing the address and length of the
98 list, along with the display line at which the palette is to be changed, to
99 ULA registers such that the ULA buffers the list and performs the redefinition
100 at the appropriate time. Throughput/bandwidth considerations might impose
101 restrictions on the practical length of such a list, however.
102
103 Palette-Free Modes
104 ------------------
105
106 Palette-free modes might be defined where bit values directly correspond to
107 the red, green and blue channels, although this would mostly make sense only
108 for modes with depths greater than the standard 4 bits per pixel, and such
109 modes would require more memory than MODE 2 if they were to have an acceptable
110 resolution.
111
112 Display Suspend
113 ---------------
114
115 Especially when writing to the screen memory, it could be beneficial to be
116 able to suspend the ULA's access to the memory, instead producing blank values
117 for all screen pixels until a program is ready to reveal the screen. This is
118 different from palette blanking since with a blank palette, the ULA is still
119 reading screen memory and translating its contents into pixel values that end
120 up being blank.
121
122 This function is reminiscent of a capability of the ZX81, albeit necessary on
123 that hardware to reduce the load on the system CPU which was responsible for
124 producing the video output.
125
126 Hardware Sprites
127 ----------------
128
129 An enhanced ULA might provide hardware sprites, but this would be done in an
130 way that is incompatible with the standard ULA, since no &FE*X locations are
131 available for allocation. In a special ULA mode, one might allocate a pair of
132 locations (for example, &FE20 and &FE21) as a pair of registers referencing a
133 region of memory from which a sprite might be found and potentially copied
134 into internal RAM, with other locations (for example, &FE22 and &FE23)
135 providing the size of the region. Alternatively, one might write the region
136 location and size through a single ULA location, with the ULA being put into a
137 particular state after each write. For example: read LSB of region, read MSB
138 of region, read size, read height.
139
140 Enhanced Graphics
141 -----------------
142
143 Screen modes with different screen memory mappings, higher resolutions and
144 larger colour depths might be possible, but this would in most cases involve
145 the allocation of more screen memory, and the ULA would probably then be
146 obliged to page in such memory for the CPU to be able to sensibly access it
147 all. Merely changing the memory mappings in order to have Archimedes-style
148 row-oriented screen addresses (instead of character-oriented addresses) could
149 be done for the existing modes, but this might not be sufficiently beneficial,
150 especially since accessing regions of the screen would involve incrementing
151 pointers by amounts that are inconvenient on an 8-bit CPU.
152
153 Enhanced Sound
154 --------------
155
156 The standard ULA reserves &FE*6 for sound generation and cassette
157 input/output, thus making it impossible to support multiple channels within
158 the given framework. The BBC Micro ULA employs &FE40-&FE4F for sound control,
159 and an enhanced ULA could adopt this interface.
160
161 Waveform Upload
162 ---------------
163
164 As with a hardware sprite function, waveforms could be uploaded or referenced
165 using locations as registers referencing memory regions.
166
167 BBC ULA Compatibility
168 ---------------------
169
170 Although some new ULA functions could be defined in a way that is also
171 compatible with the BBC Micro, the BBC ULA is itself incompatible with the
172 Electron ULA: &FE00-7 is reserved for the video controller in the BBC memory
173 map, but controls various functions specific to the 6845 video controller;
174 &FE08-F is reserved for the serial controller. It therefore becomes possible
175 to disregard compatibility where compatibility is already disregarded for a
176 particular area of functionality.
177
178 &FE20-F maps to video ULA functionality on the BBC Micro which provides
179 control over the palette (using address &FE21, compared to &FE07-F on the
180 Electron) and other system-specific functions. Since the location usage is
181 generally incompatible, this region could be reused for other purposes.