Pac-Man Emulation Guide v0.1, Oct 2008
This document is a reference for the Pac-Man video game arcade hardware, mainly written to document my findings as I create a Pac-Man emulator for learning purposes.
The information was gathered from the Internet and my own investigations and was merged and edited into this document. There are various red TODOs left in the document where I hope to add or clarify material.
This was written by Chris Lomont in Oct 2008. If you have comments or fixes then email me at (my first name) at lomont.org. Also check out my website www.lomont.org.
To start, Figure 1 shows a Pac-Man “attract” screen and the start of a level.
Figure 1 – Two views of Pac-Man
Introduction
Throughout this document decimal numbers appear as usual, and hexadecimal numbers are suffixed with an ‘h’. For example 1K is 1024 bytes = 400h bytes.
Pac-Man hardware is at the highest level:
· Main CPU : Z80A at 3.072 MHZ
· ROM: 16K of ROM in four 4K chips
· RAM: almost 2K of working RAM
· Video:
o 2K of RAM as follows
§ 1K holds tile information.
§ 1K holds per tile palette indices.
o The visible screen is 28x36 tiles.
o Each tile is an 8x8 pixel 4-color image from 16 possible colors.
o Video resolution is thus 224x288 pixels.
o Eight 16x16 pixel hardware sprites, each with a 4-color palette from 16 possible colors.
o 60.61 frames per second (60.61fps).
· Sound Chip : Custom monophonic 3-voice 4-bit Wave Sound Generator
Z80
The heart of the Pac-Man arcade machine is the 8-bit Z80 Central Processing Unit (CPU). It was released in July 1976, borrowing ideas from the Intel 8080. In fact it was designed to be binary compatible with the 8080.
The Pac-Man Z80 runs at 3.072 MHZ.
One important feature is that it addresses 16 bits of memory, so all Pac-Man addresses must lie in the range 0-65535 (0000h-FFFFh). Pac-Man actually uses less than half of this.
The Z80 also supports three interrupt modes, although the game only uses one, the so-called “mode 2” set via the Z80 instruction “IM 2.” An emulator must support port 0 in hardware, which is used to store a byte (used to form an address for an interrupt vector) which must be returned when triggering an interrupt. See the section on interrupts.
Memory Map
All hardware is memory-mapped, that is, can be accessed by reading and writing memory addresses from the CPU.
The game ROM uses the lowest 16K of memory, followed by 2K of video memory for tile selection and per tile palettes (1K each), and then a little less than 2K of general purpose RAM. This is followed at address 5000h with 256 bytes of memory-mapped hardware devices such as counters, sound hardware, and joystick switches.
|
Addresses |
Bytes |
Description |
|
0000h-3FFFh |
16,384 |
Game ROM in four 1K chips: 0000h-0FFFh pacman.6e 1000h-1FFFh pacman.6f 2000h-2FFFh pacman.6h 3000h-3FFFh pacman.6j |
|
4000h-43FFh |
1024 |
Video RAM (tile
information) |
|
4400h-47FFh |
1024 |
Video RAM (tile palettes) |
|
4800h-4FEFh |
2032 |
RAM |
|
4FF0h-4FFFh |
16 |
Sprite #, x and y flip bits (write only). 8
pairs of 2 bytes: First byte is sprite number 0-31, (bits
2-7), X flip (bit1), Y flip(bit 0) Second byte is sprite palette |
|
|
Memory Mapped Registers* |
|
|
5000h-503Fh read |
1 |
IN0 (joystick and coin slot) (each byte
returns same value) |
|
5000h write |
1 |
Interrupt
enable (bit 0: 0 = disabled, 1 = enabled) |
|
5001h write |
1 |
Sound enable (bit 0: 0 = disabled, 1 =
enabled) |
|
5002h write |
1 |
??? Aux
board enable? |
|
5003h |
1 |
Flip screen (bit 0: 0 = normal, 1 =
flipped) |
|
5004h |
1 |
1 player
start lamp (bit 0: 0 = on, 1 = off) |
|
5005h |
1 |
2 players start lamp (bit 0: 0 = on, 1 =
off) |
|
5006h |
1 |
Coin lockout
(bit 0: 0 = unlocked, 1 = locked) |
|
5007h |
1 |
Coin counter (trigger by changing bit 0
from 0 to 1) |
|
5040h-507Fh read |
64 |
IN1
(joystick and start buttons) (each byte returns same value) |
|
5040h-5045h write |
6 |
Sound voice 1 |
|
|
|
5040-5044 –
accumulator (low nibbles, used by H/W only) |
|
|
|
5045 – waveform (low nibble) |
|
5046h-504Ah write |
5 |
Sound voice
2, laid out like voice 1, missing low accumulator nibble |
|
504Bh-504Fh write |
5 |
Sound voice 3, laid out like voice 1,
missing low accumulator nibble |
|
5050h-5054h write |
5 |
Voice 1
frequency (low nibbles) |
|
5055h write |
1 |
Voice 1 volume (low nibble) |
|
5056h-505Ah write |
5 |
Voice 2
frequency and volume, laid out like voice 1 |
|
505Bh-505Fh write |
5 |
Voice 3 frequency and volume, laid out like
voice 1 |
|
5060h-506Fh write |
16 |
Sprite x, y
coordinates |
|
5080h-50BFh read |
64 |
Dip Switch Settings (each byte returns same
value) |
|
50C0h-50FF write |
64 |
Watchdog reset (each byte has the same
function) |
Table 1 - Pac-Man memory map
* Note some addresses have different read and write behavior.
ROMs
Pac-Man hardware needs various Read-Only-Memory chips to function. These hold program code, color and palette definitions, sprite and tile images, and sound waveforms. There are various sets from different manufacturers and countries.
Table 2 lists the ten ROMs needed to run Pac-Man. The filenames are from MAME[1] and come from the physical board description. These are for the Midway[2] 1980 version of Pac-Man.
|
Filename |
ROM usage |
Bytes |
Comments |
CRC[3] |
|
pacman.6e |
Code ROM1 |
4096 |
4K of code |
c1e6ab10 |
|
pacman.6f |
Code ROM2 |
4096 |
4K of code |
1a6fb2d4 |
|
pacman.6h |
Code ROM3 |
4096 |
4K of code |
bcdd1beb |
|
pacman.6j |
Code ROM4 |
4096 |
4K of code |
817d94e3 |
|
82s123.7f |
Color ROM |
32 |
32 one-byte colors |
2fc650bd |
|
82s126.4a |
Palette ROM |
256 |
64 four-byte
palettes |
3eb3a8e4 |
|
pacman.5e |
Tile ROM |
4096 |
256 8x8 pixel tile image |
0c944964 |
|
pacman.5f |
Sprite ROM |
4096 |
64 16x16
sprite images |
958fedf9 |
|
82s126.1m |
Sound ROM 1 |
256 |
8 waveforms |
a9cc86bf |
|
82s126.3m |
Sound ROM 2 |
256 |
8 waveforms |
77245b66 |
Table 2 – Pac-Man ROMs
Obtaining ROMs
These ROM sets can be found on the internet and in commercial emulators such as Microsoft Arcade Pocket Pack. Since they are copyrighted by the original owners, you should obtain a legal copy from an emulator or purchase a real Pac-Man machine from EBay! You also could purchase a legal emulator, such as the one mentioned, and strip out the ROMs.
Code ROMs
The four code ROMs are unencrypted binary Z80 code. Disassembled versions are on the web, although they are poorly commented. TODO – finish commenting disassembly and post?
Color ROM
Colors are defined in a 32-entry palette ROM (82s123.7f) where each 8-bit entry stores a 3-bit value for each of red and green and a 2-bit value for blue. The hardware maps these values through resistors of value 1000s ohm, 470 ohms, and 220 ohms, dropping the 1000 ohm resistor for blue[4]. Thus Pac-Man hardware can use 32 colors, however the ROM (and hence the game) only contains 16 colors in the lower 16 bytes; the upper 16 bytes are zeros for all black.
Each color entry corresponds to color intensity on the output hardware. For a fixed voltage V, current (giving intensity) is proportional to 1/Resistance. The maximum current is then
The highest intensity, 3967, is scaled to the highest color-channel value FFh. Noting
then the 220 ohm resistor (and the
corresponding bit) gives a weight of 2350 * FFh/3967 =97h. Similarly 470 ohm
gives 47h and 1000 gives 21h.
Using this to compute the output RGB values gives the weights 97h, 47h, and 21h. Since the blue channel only has 2 bits with 470 and 220 ohm resistors, similar calculations give weights 51h and AEh respectively. This is summarized in Table 3.
|
Bit |
Color |
Connected to resistor |
Weight |
|
0 |
Red (Least amount) |
1000 ohm |
21h |
|
1 |
Red |
470 ohm |
47h |
|
2 |
Red (Most amount) |
220 ohm |
97h |
|
3 |
Green (Least
amount) |
1000 ohm |
21h |
|
4 |
Green |
470 ohm |
47h |
|
5 |
Green (Most
amount) |
220 ohm |
97h |
|
6 |
Blue |
470 ohm |
51h |
|
7 |
Blue |
220 ohm |
AEh |
Table 3 – Color ROM bits
The first byte in the ROM is color 0, the second byte is color 2, etc. Although the ROM has space for 32 colors, only the first 16 are used, with the remaining 16 bytes being 0 which is black. The first 16 colors are shown in Figure 2, and the corresponding RGB values (0-255 range) are
Figure 2 – Pac-Man Colors
For some reason four of the entries are black[5].
Palette ROM
Each palette in the 256 byte palette ROM (82s126.4a) consists of 4 bytes, so this ROM stores 64 palettes. The first four bytes are palette 0, the next four bytes are palette 1, etc. The values in each entry reference a color from the color ROM. Since the color ROM only uses 16 colors, each byte only uses the lower 4 bits to store a number 0-15. Finally, only the first 32 palettes are not completely black, and of these several are all black.
Figure 3 shows the first 32 palettes, left to right, one palette per column. Palette color 0 is the bottom and palette color 3 is the top. By inspection the ghost palettes are 1, 3, 5, and 7.
Figure 3 – Pac-Man Palettes
Tile (Character) ROM
The background, scores, bonus items, and dots are drawn on a static background using images from the 4096 byte tile ROM (pacman.5e). Figure 4 shows the images decoded from the tile ROM using palette 1.
Each tile is an 8x8 pixel image, stored as 2 bits per pixel, thus using 16 bytes per tile. The 4096 byte ROM thus holds 256 tiles, including a character font and two sets of differently spaced digits.
Drawing a tile uses a byte in Tile RAM to select one of the 256 four-color tiles to display, and also uses a byte in the Palette RAM to select the palette used to color the tile.
Decoding images from the Tile ROM is slightly messy. Each byte stores 4 pixels, but they are stored in bitplanes. Thus the least significant bit of each pixel is stored in the lower four bits of the byte, and the most significant bit of each pixel is stored in the higher 4 bits of the byte (see Table 4).
|
Bit |
Usage |
|
0 |
Bit 0 of pixel #1 |
|
1 |
Bit 0 of
pixel #2 |
|
2 |
Bit 0 of pixel #3 |
|
3 |
Bit 0 of
pixel #4 |
|
4 |
Bit 1 of pixel #1 |
|
5 |
Bit 1 of
pixel #2 |
|
6 |
Bit 1 of pixel #3 |
|
7 |
Bit 1 of pixel #4 |
Table 4 – Tile ROM byte layout
The resulting 4 pixels from each byte are vertical strips. The first 8 bytes draw the lower half of the tile, left to right, top to bottom. The last 8 bytes defining the tile draw the top half of the tile[6]. The images are stored in columns since the Pac-Man hardware has the screen rotated 90 degrees, and reading them in this order results in easier scanline drawing in hardware (TODO - conjecture!?).
Figure 4 – Pac-Man tiles in the tile ROM
Sprite ROM
The 4096 byte Sprite ROM (pacman.5f) stores 16x16 pixel sprites. Each pixel uses 2 bits, resulting in each sprite using 64 bytes of ROM. 4096/64 = 64 sprites stored in ROM. Figure 5 shows the decoded sprites with sprites 0-7 across the top, etc. The image is drawn using palette 1 which is black, light gray, blue, and red.
Figure 5 – Pac-Man sprite ROM
Decoding the sprites works similarly to decoding the tiles. Each byte represents 4 pixels in a column, stored low bits then high bits just like the tiles. Each 8 bytes draw a strip of 8x4 pixels, just like tiles. These 8 strips are then arranged
|
5 |
1 |
|
6 |
2 |
|
7 |
3 |
|
4 |
0 |
The complete pixel ordering is then[7]
Sound ROMs
Sound is generated by playing waveforms stored in the two 256 byte Sound ROMs (82s126.1m and 82s126.3m). The sound hardware only takes 4-bit samples, so each byte entry has the top nibble set to 0. This gives in total 512 four-bit sound samples. These are organized into 16 waveforms, each 32 samples long, and each sample value from 0-15.
Ordering them from Sound ROM1 (82s126.1m) to Sound ROM2 (82s126.3m), the waveforms are shown in Figure 6.
Figure 6 – Pac-Man Sound ROM waveforms
These are ordered 0-3 in the top row, left to right. The next row left to right is 4,5,6,7, etc. It seems the last four are not used, and the previous four may also be unused[8].
Video
The video is drawn from two components: tiles and sprites. The hardware supports
· 8 sprites that are 16x16 pixel images,
· a 28 wide by 36 tall grid of tiles, each capable holding an 8x8 pixel image.
Thus the effective pixel resolution is 224x288 (28*8 = 224, 36*8=288). The tiles are used to draw a static background, and the sprites are used to draw moving items.
Layout
To understand how video is drawn, the mapping from memory address to screen locations is needed. The following description works for both the 1K tile RAM 4000h-43FFh (each byte selects a tile to draw) and the 1K palette RAM 4400h-47FFh.
The first 64 addresses draw two rows, right to left, top to bottom, along the bottom of the screen, each row 32 tiles wide, with 4 of the tiles off-screen on both ends. Then the next 380h (896 decimal) addresses fill in the main screen, top to bottom, right to left, using offsets from 40h to 3BFh. This gives a 28x32 tile center region. Finally the last 64 addresses in each bank draw two more 32 tile wide rows along the top, right to left, top to bottom, again with 4 tiles off-screen. The off-screen locations could be used to hide unused sprites, which are always on[9]. The result is a 28x36 tile visible screen. With each tile 8x8 pixels, this gives the 224x288 pixel resolution.
For clarity examine Figure 7 and Figure 8 for offsets. Figure 9 shows an overlaid Pac-Man screen.
|
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 |
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 |
3DF 3DE 3DD 3DC 3DB 3DA 3D9 3D8 3D7 3D6 3D5 3D4 3D3 3D2 3D1 3D0 3CF 3CE 3CD 3CC 3CB 3CA 3C9 3C8 3C7 3C6 3C5 3C4 3C3 3C2 3C1 3C0 3FF 3FE 3FD 3FC 3FB 3FA 3F9 3F8 3F7 3F6 3F5 3F4 3F3 3F2 3F1 3F0 3EF 3EE 3ED 3EC 3EB 3EA 3E9 3E8 3E7 3E6 3E5 3E4 3E3 3E2 3E1 3E0 3A0 380 360 340 320 300 2E0 2C0 2A0 280 260 240 220 200 1E0 1C0 1A0 180 160 140 120 100 0E0 0C0 0A0 080 060 040 3A1 381 361 341 321 301 2E1 2C1 2A1 281 261 241 221 201 1E1 1C1 1A1 181 161 141 121 101 0E1 0C1 0A1 081 061 041 3A2 382 362 342 322 302 2E2 2C2 2A2 282 262 242 222 202 1E2 1C2 1A2 182 162 142 122 102 0E2 0C2 0A2 082 062 042 3A3 383 363 343 323 303 2E3 2C3 2A3 283 263 243 223 203 1E3 1C3 1A3 183 163 143 123 103 0E3 0C3 0A3 083 063 043 3A4 384 364 344 324 304 2E4 2C4 2A4 284 264 244 224 204 1E4 1C4 1A4 184 164 144 124 104 0E4 0C4 0A4 084 064 044 3A5 385 365 345 325 305 2E5 2C5 2A5 285 265 245 225 205 1E5 1C5 1A5 185 165 145 125 105 0E5 0C5 0A5 085 065 045 3A6 386 366 346 326 306 2E6 2C6 2A6 286 266 246 226 206 1E6 1C6 1A6 186 166 146 126 106 0E6 0C6 0A6 086 066 046 3A7 387 367 347 327 307 2E7 2C7 2A7 287 267 247 227 207 1E7 1C7 1A7 187 167 147 127 107 0E7 0C7 0A7 087 067 047 3A8 388 368 348 328 308 2E8 2C8 2A8 288 268 248 228 208 1E8 1C8 1A8 188 168 148 128 108 0E8 0C8 0A8 088 068 048 3A9 389 369 349 329 309 2E9 2C9 2A9 289 269 249 229 209 1E9 1C9 1A9 189 169 149 129 109 0E9 0C9 0A9 089 069 049 3AA 38A 36A 34A 32A 30A 2EA 2CA 2AA 28A 26A 24A 22A 20A 1EA 1CA 1AA 18A 16A 14A 12A 10A 0EA 0CA 0AA 08A 06A 04A 3AB 38B 36B 34B 32B 30B 2EB 2CB 2AB 28B 26B 24B 22B 20B 1EB 1CB 1AB 18B 16B 14B 12B 10B 0EB 0CB 0AB 08B 06B 04B 3AC 38C 36C 34C 32C 30C 2EC 2CC 2AC 28C 26C 24C 22C 20C 1EC 1CC 1AC 18C 16C 14C 12C 10C 0EC 0CC 0AC 08C 06C 04C 3AD 38D 36D 34D 32D 30D 2ED 2CD 2AD 28D 26D 24D 22D 20D 1ED 1CD 1AD 18D 16D 14D 12D 10D 0ED 0CD 0AD 08D 06D 04D 3AE 38E 36E 34E 32E 30E 2EE 2CE 2AE 28E 26E 24E 22E 20E 1EE 1CE 1AE 18E 16E 14E 12E 10E 0EE 0CE 0AE 08E 06E 04E 3AF 38F 36F 34F 32F 30F 2EF 2CF 2AF 28F 26F 24F 22F 20F 1EF 1CF 1AF 18F 16F 14F 12F 10F 0EF 0CF 0AF 08F 06F 04F 3B0 390 370 350 330 310 2F0 2D0 2B0 290 270 250 230 210 1F0 1D0 1B0 190 170 150 130 110 0F0 0D0 0B0 090 070 050 3B1 391 371 351 331 311 2F1 2D1 2B1 291 271 251 231 211 1F1 1D1 1B1 191 171 151 131 111 0F1 0D1 0B1 091 071 051 3B2 392 372 352 332 312 2F2 2D2 2B2 292 272 252 232 212 1F2 1D2 1B2 192 172 152 132 112 0F2 0D2 0B2 092 072 052 3B3 393 373 353 333 313 2F3 2D3 2B3 293 273 253 233 213 1F3 1D3 1B3 193 173 153 133 113 0F3 0D3 0B3 093 073 053 3B4 394 374 354 334 314 2F4 2D4 2B4 294 274 254 234 214 1F4 1D4 1B4 194 174 154 134 114 0F4 0D4 0B4 094 074 054 3B5 395 375 355 335 315 2F5 2D5 2B5 295 275 255 235 215 1F5 1D5 1B5 195 175 155 135 115 0F5 0D5 0B5 095 075 055 3B6 396 376 356 336 316 2F6 2D6 2B6 296 276 256 236 216 1F6 1D6 1B6 196 176 156 136 116 0F6 0D6 0B6 096 076 056 3B7 397 377 357 337 317 2F7 2D7 2B7 297 277 257 237 217 1F7 1D7 1B7 197 177 157 137 117 0F7 0D7 0B7 097 077 057 3B8 398 378 358 338 318 2F8 2D8 2B8 298 278 258 238 218 1F8 1D8 1B8 198 178 158 138 118 0F8 0D8 0B8 098 078 058 3B9 399 379 359 339 319 2F9 2D9 2B9 299 279 259 239 219 1F9 1D9 1B9 199 179 159 139 119 0F9 0D9 0B9 099 079 059 3BA 39A 37A 35A 33A 31A 2FA 2DA 2BA 29A 27A 25A 23A 21A 1FA 1DA 1BA 19A 17A 15A 13A 11A 0FA 0DA 0BA 09A 07A 05A 3BB 39B 37B 35B 33B 31B 2FB 2DB 2BB 29B 27B 25B 23B 21B 1FB 1DB 1BB 19B 17B 15B 13B 11B 0FB 0DB 0BB 09B 07B 05B 3BC 39C 37C 35C 33C 31C 2FC 2DC 2BC 29C 27C 25C 23C 21C 1FC 1DC 1BC 19C 17C 15C 13C 11C 0FC 0DC 0BC 09C 07C 05C 3BD 39D 37D 35D 33D 31D 2FD 2DD 2BD 29D 27D 25D 23D 21D 1FD 1DD 1BD 19D 17D 15D 13D 11D 0FD 0DD 0BD 09D 07D 05D 3BE 39E 37E 35E 33E 31E 2FE 2DE 2BE 29E 27E 25E 23E 21E 1FE 1DE 1BE 19E 17E 15E 13E 11E 0FE 0DE 0BE 09E 07E 05E 3BF 39F 37F 35F 33F 31F 2FF 2DF 2BF 29F 27F 25F 23F 21F 1FF 1DF 1BF 19F 17F 15F 13F 11F 0FF 0DF 0BF 09F 07F 05F 01F 01E 01D 01C 01B 01A 019 018 017 016 015 014 013 012 011 010 00F 00E 00D 00C 00B 00A 009 008 007 006 005 004 003 002 001 000 03F 03E 03D 03C 03B 03A 039 038 037 036 035 034 033 032 031 030 02F 02E 02D 02C 02B 02A 029 028 027 026 025 024 023 022 021 020 |
Figure 7 – Pac-Man video address offsets
Figure 8 –Pac-Man Video addresses
Figure 9 – Pac-Man screen overlaid on video addresses
Tiles
Each byte in Video RAM entry from 4000h-43FFh (Tile RAM) causes a tile from the Tile ROM to be drawn. The corresponding byte in 4400h-47FFh (Palette RAM) selects a palette from the palette ROM to apply to the tile, and this palette in turn references colors defined in the color ROM.
Each tile is an 8x8 pixel image; the resulting grid is a
28x36 grid of tiles. The first two rows, 32 bytes each, are used to hold the
fruit/credit/prize display at the screen bottom. The next 28 rows are rotated
as explained above, and contain the maze and game play area. The top two rows
contain the score and related information.
Sprites
The Pac-Man hardware supports 8 sprites; each is a 16x16 pixel image that can be drawn at any location. Each can be flipped in the x and/or y directions, reducing the amount of sprite images stored in ROM. For example, the left and right facing Pac-Man images are the same sprite with a different x-flip bit set. The cocktail mode Pac-Man games flip the screen for two-player modes by flipping sprites and reordering tiles.
The eight hardware sprites are programmed through addresses 4FF0h-4FFFh and 5060h-506Fh, listed in Table 5. For each of the 8 sprites, the sprite image number 0-63, x-flip, y-flip, and palette are set through the addresses shown in Table 6.
|
Sprite |
X-location |
Y-location |
Sprite#, flip-x, flip-y |
Palette |
|
0 |
5060h |
5061h |
4FF0h |
4FF1h |
|
1 |
5062h |
5063h |
4FF2h |
4FF3h |
|
2 |
5064h |
5065h |
4FF4h |
4FF5h |
|
3 |
5066h |
5067h |
4FF6h |
4FF7h |
|
4 |
5068h |
5069h |
4FF8h |
4FF9h |
|
5 |
506Ah |
506Bh |
4FFAh |
4FFBh |
|
6 |
506Ch |
506Dh |
4FFCh |
4FFDh |
|
7 |
506Eh |
506Fh |
4FFEh |
4FFFh |
Table 5 – Pac-Man sprite addresses
|
Byte 0 |
|
Byte 1 |
||||||||||||||
|
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
Sprite # 0-63 |
X flip |
Y flip |
|
Palette entry 0-15 |
||||||||||||
Table 6 – Sprite register definition
Each sprite can thus select a different palette. Color 0 in sprite images are drawn as transparent, allowing the background tiles to show through. Setting the X and Y locations allows the sprites to be positioned anywhere on the screen with pixel accuracy. They’re positioned offscreen when not needed. Sprites are displayed in reverse order; higher numbered overlapped sprites are overwritten by lower numbered sprites.
The X and Y locations denote the upper left corner of the sprite image. The screen coordinates start on the lower right corner of the screen. Thus, including hidden coordinates, the lowest coordinate pair where a 16x16 sprite is completely visible is location (31,16). If drawn lower, it wraps to the top of the screen. The sprite does not show up on the top or bottom two rows where the video is laid out differently, as explained the section on Video Layout. Visible pixel coordinates are in Figure 10.
Figure 10 – Screen coordinates Vertical wrap visible Vertical wrap visible (239,256) (239,16) (31,16) (31,256)
Sound
The sound chip is a custom-made 3-channel Waveform Sound Generator (WSG). It can play three simultaneous voices each with independent volume, frequency, and waveform. It is accessed using 4-bit memory-mapped registers, whose addresses are in Table 7. The WSG runs at main CPU clock/32, i.e., 3.072 MHz/32 = 96 kHz.
|
Register |
Address |
Notes |
|
Voice 1 Waveform |
5045h |
low 3 bits used – selects waveform 0-7 from
ROM |
|
Voice 1 Frequency |
5050h-5054h |
20 bits in
low nibbles |
|
Voice 1 Volume |
5055h |
low nibble – 0 off to 15 loudest |
|
Voice 1 Accumulator |
5040h-5044h |
low nibbles,
used by H/W only |
|
|
|
|
|
Voice 2 Waveform |
504Ah |
low 3 bits
used – selects waveform 0-7 from ROM |
|
Voice 2 Frequency |
5056h-5059h |
16 bits in low nibbles |
|
Voice 2 Volume |
505Ah |
low nibble –
0 off to 15 loudest |
|
Voice 2 Accumulator |
5046h-5049h |
low nibbles, used by H/W only |
|
|
|
|
|
Voice 3 Waveform |
504Fh |
low 3 bits used – selects waveform 0-7 from
ROM |
|
Voice 3 Frequency |
505Bh-505Eh |
16 bits in
low nibbles |
|
Voice 3 Volume |
505Fh |
low nibble – 0 off to 15 loudest |
|
Voice 3 Accumulator |
504Bh-504Eh |
low nibbles, used by H/W only |
Table 7 – Pac-Man sound registers.
Each register disregards the top nibble in each byte. The hardware supports 3 monophonic voices, each with 16 volume levels and an independent choice of 8 pre-defined waveforms. One voice has 20 bits of frequency choice, while the other two have 16 bits of frequency.
At each clock and for each voice the hardware performs the following steps:
1. Add the 20-bit (or 16-bit) voice frequency to the 20-bit (or 16-bit) accumulator.
2. Use the waveform 0-7 to lookup a 32-byte sample in the Sound ROM.
3. Take the top 5 bits of the accumulator to look up a nibble 0-31 in that sample.
4. Multiply that nibble by the volume nibble 0-15.
5. Send the result to the amplifier for output.
As a result, a voice is silent if the volume is zero.
Frequencies and counters are 20-bit or 16-bit values, depending on voice, and are stored in the least significant nibble of 5 (or 4) bytes, with the least significant nibble in the lowest address. Voices #2 and #3 do not implement the lowest nibble, which is always 0. Thus they have slightly lower dynamic range than voice #1, which is used in the code for TODO.
Waveforms
As explained in the Sound ROM section, samples are stored as
32 entries of 4-bits each. They can be selected by setting 0-7 in the
appropriate voice.
Frequency
For each voice there are 4 registers controlling the tone. Voice #1 adds a 5th register allowing very low frequencies.
How does this convert to notes for a musical scale? Using scientific pitch notation[10] to denote middle C as C4, then the first A higher in pitch is known as A440 (or as A4) since it is 440 Hz. Here’s a computation of the values to put in the frequency register to get a 440 Hz tone for Voice #1.
To play the A4 tone (440 Hz) use sample 0, which is a pure sine tone and has one cycle per sample. If a sample has two cycles per sample this example would need changed accordingly.
Starting with the 96 kHz accumulator rate, sample 0 produces a complete sound cycle every 32 samples. To get 440 Hz, 440*32 = 14080 samples are needed per second. 96000 clocks per second / 14080 samples per second = 75/11 clocks per sample. Since the top 5 bits of 20 select the sample, there are 20-5 = 15 bits representing “fractional” values. 2^15/(75/11) = 4806 = 12C6h.This needs stored in the frequency of a voice to use sample 0 for a note with A4 = 440 Hz frequency.
Thus to get the register value 
for frequency 
using sample 0 use the formula
Each octave is half/double the frequency of the one above/below
it, so this gives counts for all octaves of A. Since there are 12 half-steps
per octave, to go one half-step higher simple multiply the value by 
.
C3 is 3 half-steps up, so has frequency 
Hz. This then corresponds to 
1653h.
Filling a note table gives Table 8.
|
|
Octave |
|||||||||
|
Note |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
|
C |
B3h |
165h |
2CAh |
595h |
B2Ah |
1653h |
2CA7h |
594Dh |
B29Ah |
16535h |
|
C#/Db |
BDh |
17Ah |
2F5h |
5EAh |
BD4h |
17A7h |
2F4Eh |
5E9Dh |
BD39h |
17A72h |
|
D |
C8h |
191h |
322h |
644h |
C88h |
190Fh |
321Eh |
643Dh |
C87Ah |
190F3h |
|
D#/Eb |
D4h |
1A9h |
352h |
6A3h |
D46h |
1A8Dh |
3519h |
6A33h |
D465h |
1A8CBh |
|
E |
E1h |
1C2h |
384h |
708h |
E10h |
1C21h |
3842h |
7083h |
E107h |
1C20Dh |
|
F |
EEh |
1DDh |
3BAh |
773h |
EE7h |
1DCDh |
3B9Ah |
7734h |
EE68h |
1DCD0h |
|
F#/Gb |
FDh |
1F9h |
3F2h |
7E5h |
FC9h |
1F93h |
3F25h |
7E4Bh |
FC95h |
1F92Ah |
|
G |
10Ch |
217h |
42Eh |
85Dh |
10BAh |
2173h |
42E7h |
85CDh |
10B9Ah |
21734h |
|
G#/Ab |
11Ch |
237h |
46Eh |
8DCh |
11B8h |
2370h |
46E1h |
8DC2h |
11B84h |
23708h |
|
A |
12Ch |
259h |
4B1h |
963h |
12C6h |
258Ch |
4B18h |
9630h |
12C60h |
258BFh |
|
A#/Bb |
13Eh |
27Ch |
4F9h |
9F2h |
13E4h |
27C8h |
4F8Fh |
9F1Eh |
13E3Ch |
27C78h |
|
B |
151h |
2A2h |
545h |
A89h |
1513h |
2A25h |
544Ah |
A894h |
15128h |
2A251h |
Table 8 – Note frequency chart
Note that this is for the 5 register
voices. For the 4 register voice round the low nibble up; this will result in
tones being slightly off.
Volume
Per-voice volume is a straightforward value 0-15, with 0 being muted to 15 being loudest.
Accumulator
The per-voice accumulator is used internally to select Sound ROM samples at some program defined frequency. It is not clear if these can be read (or set?) from the code (TODO – find out).
Sound Enable Register
Register 50C0h is the sound enable register, enabling or
disabling all voices simultaneously. Write 0 to bit 0 of this register to
disable sound and write 1 to bit 0 to enable sound.
Code
Sound register usage can be studied from the Pac-Man ROM sound routines through disassembly (if you know how, can read Z80, etc.). The main sound code is at addresses 2CC1h-2F54h, the interrupt portion is in two sections at 009Dh-00D2h (VBLANK1?) and 01B9h-01BCh (VBLANK2). Sound tables are at 3B30h-3CDDh. Details are at http://www.vecoven.com/elec/pacman/code/pac_z80.html.
Some routine highlights:
The software updates the sound registers 60 times a second during the VBLANK. Each voice plays an effect or a song.
An effect is encoded with eight bytes, and can be played on any voice.
0. Upper 3 bits are frequency shift, lower 3 bits are sample select.
1. Initial base frequency
2. Frequency increment
3. Upper bit causes reversal, lower 7 bits is duration.
4. Frequency increment added to base when repeat > 1
5. Repeat
6. Upper 4 bits denote volume adjust type, lower 4 bits is volume
7. Volume increment.
A song is a list of special bytes and tone information, and is played on voices 1 and 2 simultaneously.
A regular byte contains
· Upper 3 bits – log base 2 of duration
· Lower 4 bits – base frequency using lookup table
· Sometimes the lower 5 bits are assigned to W_DIR where the 5th bit has significance?!
Special song bytes include
· F0h followed by 2 bytes : address where song continues (jump in ROM)
· F1h followed by 1 byte : wave select
· F2h followed by one byte: frequency increment
· F3 followed by one byte: volume
· F4h followed by 1 byte : type
· FFh : end of song.
Interrupts
Interrupt Enable
Address 5000h is the interrupt enable register. Setting bit
0 to 0 disabled vertical blank (VBLANK) interrupts, and setting bit 0 to 1
enables VBLANK interrupts.
VBLANK
This is the vertical blank signal, which is triggered each time the electron gun scans to the bottom of the screen while drawing an image. This happens 60.61 times a second, which is every 16.67 milliseconds. If int