Z80 Assembly programming for the Amstrad CPC
| The CPC was the 8 bit I grew up
with... while slower in some ways than the C64, it had far superior
graphical capabilities to the ZX Spectrum, and usually beats the MSX
for graphical speed because of it's smaller screen footprint (16k on
the CPC to 24k on the MSX) and its CRTC graphics chip is favoured by
the modern clever demo authors... While the budget tape-based 464 machine had 64k, the disk system - the 6128 - has 128k and a disk system... and after market upgrades can give the machine up to 576k There were two major generations of the CPC, the regular version, and the version released in the 90... the CPC+, which has has hardware sprites, enhanced color |
 |
 |
 |
|
 |
ChibiAkumas Tutorials:
| Register | Meaning | Bit Meaning | Details |
| 0 | Tone Pitch L - Channel A | LLLLLLLL | Lower value = Higher pitch |
| 1 | Tone Pitch H - Channel A | ----HHHH | Lower value = Higher pitch |
| 2 | Tone Pitch L - Channel B | LLLLLLLL | Lower value = Higher pitch |
| 3 | Tone Pitch H - Channel B | ----HHHH | Lower value = Higher pitch |
| 4 | Tone Pitch L - Channel C | LLLLLLLL | Lower value = Higher pitch |
| 5 | Tone Pitch H - Channel C | ----HHHH | Lower value = Higher pitch |
| 6 | Noise Generator | ---NNNNN | Higer = Faster noise |
| 7 | Mixer | --NNNTTT | N=Noise T=Tone (Channel --CBACBA 1=mute 0=normal) |
| 8 | Amplitude - Channel A | ---EVVVV | E=Envelope (1=Enabled) VVVV=Volume |
| 9 | Amplitude - Channel B | ---EVVVV | E=Envelope (1=Enabled) VVVV=Volume |
| 10 | Amplitude - Channel C | ---EVVVV | E=Envelope (1=Enabled) VVVV=Volume |
| 11 | Envelope L (Volume over time) | LLLLLLLL | Lower=Faster Envelope |
| 12 | Envelope H (Volume over time) | HHHHHHHH | Lower=Faster Envelope |
| 13 | Envelope Selection | ----EEEE | Envelope number (See PDF) |
Gate Array:
The Gate array is at port &7Fxx... It has multiple purposes depending on the top two bits passed in the C byte
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | Name | Bit meanings |
| 0 | 0 | - | B | P | P | P | P | Rom/Mode | B=Border P=Pen (0-3 / 0-15) |
| 0 | 1 | - | C | C | C | C | C | Palette color selection | C= Clolor number (0-26) |
| 1 | 0 | - | I | H | L | M | M | Rom / Mode | I= Interrupt mode H=High rom bank L=Low rom bank M=screen mode |
| 1 | 1 | B | B | B | R | R | R | Ram Bank | R = Ram config B= Bank number (0=128k 1+2=256k etc) |
CRTC
The CRTC registers are selected with &BCxx (where xx is the register number)... and set with &BDxx (where xx is the new value
| Hight Byte Value | F | E | D | C | B | A | 9 | 8 | Name |
| &BC | 1 | 0 | 1 | 1 | 1 | 1 | 0 | 0 | Register Select |
| &BD | 1 | 0 | 1 | 1 | 1 | 1 | 0 | 1 | Register Write |
CRTC - Registers
CRTC registers will configure the Logical screen we draw on, and the physical screen shown by the Analog monitor...
You'll want to leave many of the settings alone, as they'll just result in a corrupt screen you can't view!
| Reg Num | Name | Range | Bits | Default
Value |
Speccy 256x192 |
Overscaln 384x272(26k) |
Details |
| &00 | Horizontal Total | 0-255 | DDDDDDDD | 63 | 63 | 63 | Physical width of screen – Leave alone! |
| &01 | Horizontal Displayed | 0-255 | DDDDDDDD | 40 | 32 | 48 | Logical width in Chars (8 pixels in mode 1) |
| &02 | Horizontal Sync Position | 0-255 | DDDDDDDD | 46 | 42 | 51 | Logical Xpos |
| &03 | Horizontal and Vertical Sync Widths | 0-15,0-15 | VVVVHHHH | 142 | 134 | 142 | Physical width of screen – Leave alone! |
| &04 | Vertical Total | 0-127 | -DDDDDDD | 38 | 38 | 38 | Physical height of screen – Leave alone! |
| &05 | Vertical Total Adjust | 0-31 | ---DDDDD | 0 | 0 | 0 | Scanline Offset |
| &06 | Vertical Displayed | 0-127 | -DDDDDDD | 25 | 24 | 34 | Logical Height in Chars (8 Pixels) |
| &07 | Vertical Sync position | 0-127 | -DDDDDDD | 30 | 31 | 35 | Logical Ypos of screen |
| &08 | Interlace and Skew | 0-3 | ------DD | 0 | 0 | 0 | 0/2=off 1/3=on – Leave alone! |
| &09 | Maximum Raster Address | 0-31 | ---DDDDD | 7 | 7 | 7 | Max Raster Address – Leave alone! |
| &0A | Cursor Start Raster | 0-127 | -DDDDDDD | 0 | 0 | 0 | |
| &0B | Cursor End Raster | 0-31 | ---DDDDD | 0 | 0 | 0 | |
| &0C | Display Start Address (H) | 0-63 | xxPPSSOO | 00 / 16 /
32 / 48 |
00 / 16 /
32 / 48 |
12+1 / 28 / 44+1 / 60 |
PP=Screen
Page (11=C000) S=Size(11=32k else 16k) O=Offset |
| &0D | Display Start Address (L) | 0-255 | OOOOOOOO | 0 | 0 | 0 | O=Offset |
| &0E | Cursor Address (H) | 0-63 | c--DDDDDD | 0 | 0 | 0 | |
| &0F | Cursor Address (L) | 0-255 | DDDDDDDD | 0 | 0 | 0 | |
| &10 | Light Pen Address (H) | 0-63 | --DDDDDD | 0 | 0 | 0 | Read Only |
| &11 | Light Pen Address (L) | 0-255 | DDDDDDDD | 0 | 0 | 0 | Read Only |
CPC Plus hardware sprites!
The Amstrad CPC has 16 hardware sprites... each is 16x16
CPC+ sprites have a separate 16 colors to the normal palette, also they are always 16 color, even in mode 1!
CPC+ Sprite colors (-GRB) are defined by the range &6422-&643F
| Sprite Number | DataAddr (256bytes) | Xpos (2bytes) | Ypos (2bytes) | Resolution (1byte) |
| 1 | &4000 | &6000 | &6002 | &6004 |
| 2 | &4100 | &6008 | &600A | &600C |
| 3 | &4200 | &6010 | &6012 | &6014 |
| 4 | &4300 | &6018 | &601A | &601C |
| 5 | &4400 | &6020 | &6022 | &6024 |
| 6 | &4500 | &6028 | &602A | &602C |
| 7 | &4600 | &6030 | &6032 | &6034 |
| 8 | &4700 | &6038 | &603A | &603C |
| 9 | &4800 | &6040 | &6042 | &6044 |
| 10 | &4900 | &6048 | &604A | &604C |
| 11 | &4A00 | &6050 | &6052 | &6054 |
| 12 | &4B00 | &6058 | &605A | &605C |
| 13 | &4C00 | &6060 | &6062 | &6064 |
| 14 | &4D00 | &6068 | &606A | &606C |
| 15 | &4E00 | &6070 | &6072 | &6074 |
| 16 | &4F00 | &6078 | &607A | &607C |
DataAddr is the pointer to the sprite data... note only 4 bits of each byte are used (----CCCC).... so 16x16=256 bytes
Xpos is 2 bytes, in little endian format... screen co-ordinates are based on mode 2 - so xpos should be between -64 to +639 (0 is leftmost visible pixel)
Ypos is 2 bytes, in little endian format... Ypos should be between -64 to +200 (0 is topmost visible pixel)
Resolution is defined by a single byte:
PP
Each (X,Y) size can be 0-3... 0 will mean the sprite isn't shown, 1,2,3 are magnifications 1,2 and 4.... however please note, these are relative to Mode 2... so 'square' pixels are defined by %00001001 = 9
This is why You'll see in ChibiAkumas, sprites are set to 'on' with a value of 9, or 'off' with a value of 0
Amstrad Links
Winape - Not just the easiest to use CPC emulator, but the easiest Z80 platform for beginner ASM programmers!
www.cantrell.org.uk - A great source of CPC and ASM info.. My cheatsheet ASM list is based on the one from cantrell.org.uk
CRTC - Details of the amstrad CPC CRTC hardware
Amstrad Firmware guide - Pdf documenting the CPC firmware calls
CPC Firmware Guide - Detailed info on how the CPC hardware and firmware
Basic Manual - You'll want to know at least enough basic to do calls and operate the computer
CpcWiki - Web community full of helpful people!
General Z80 Assembly Tutorials:
B. Beginner series - Learn the basics
A. Advanced series - In more detail
M. Multiplatform series - programming methods that work on all systems
| View Options |
| Default Dark |
| Simple (Hide this menu) |
| Print Mode (white background) |
| Top Menu |
| Youtube channel |
| ASM Programming Forums |
| GitHub |
| Dec/Bin/Hex/Oct/Ascii Table |
| 68000 Content |
Learn 68000 Assembly  |
| Platform Specific Series |
| Grime 68000 |
| 68000 Downloads |
| 68000 Cheatsheet |
| Sources.7z |
| DevTools kit |
| 68000 Platforms |
| Amiga 500 |
| Atari ST |
| Neo Geo |
| Sega Genesis / Mega Drive |
| Sinclair QL (Quantum Leap) |
| X68000 (Sharp x68k) |
| My Game projects |
| Chibi Aliens |
| Chibi Akumas |
| Misc bits |
| Ruby programming |
Chibi Akumas V1.666 has taken over 350 hours of development, if you want to support my work, and learn all the secrets of the game's development, please back me on patreon!
Thanks to Homebrew Legends for help promoting my game!
Buy ChibiAkuma(s) games now!