- 1 Introduction
- 2 ZX Spectrum
- 2.1 Setting up
- 2.2 Start values
- 2.3 Video display
- 2.4 Memory Configuration
- 2.5 Sound
- 2.6 Small demos with documented source code
- 2.7 Additional Resources
Wanting to start sizecoding on a Z80 platform in this day and age can be tough.
So here is a bit of help to get you started:
The Z80 can be seen as the little 8-bit brother of X86 chipsets, with many similarities. If you are coming from a X86 background, this might help you get a bit more grip on the Z80. These are the register pairs of the Z80, as seen from a X86 programmers perspective.
- AF = AL + Flags
- HL = Can be seen as BX (H=BH,L=BL) or SI in a (HL) setting, like BX also used for addressing.
- BC = Can be seen as CX (B=CH,C=CL), often used for loops
- DE = Can be seen as DX (D=DH,E=DL) or DI in a (DE) setting
- IX = 16 bit Index Register X, can also be accessed with IXH,IXL
- IY = 16 bit Index Register Y, can also be accessed with IYH,IYL
For each of the main registers there also exists a shadow register. These cannot be accessed directly, but must be swapped in and out with the main register set. The shadow registers are usually denoted by the ' symbol. They can be swapped with the following commands:
- EX AF,AF' = Swaps AF with AF'
- EXX = Swaps BC, DE and HL with BC', DE' and HL'
There are no shadow registers for the index registers.
Note: For a lot of operations, you can only use the A(8bit) and HL(16bit) registers. The Sjasmplus assembler has extra syntax and fake-instructions support which may produce unexpected results when source contains other than official Zilog syntax (but the parser can be configured to work in more relaxed way allowing more variations in syntax).
Here is a rough translation for some of the Z80 instructions:
- BIT = TEST
- CP = CMP (although the Z80 has many other handy compare functionality)
- DJNZ = LOOP (decreases B and checks not zero)
- EXE = Exchange all registers with Shadow registers, can be used a bit like PUSHA/POPA
- EX = XCHG
- HALT = HLT
- JP = JMP
- JR = JMP NEAR (Jump Relative)
- LD = MOV
- LDI = MOVSB (tmp=(HL),(DE)=tmp, DE++, HL++)
- LDIR = REP MOVSB (tmp=(HL),(DE)=tmp, DE++, HL++, BC--)
Learning Z80 Assembler
There are many Z80 tutorials available online, but one i found very simple and clear is at this 1996 styled webpage ;-)
There is no proper index-page for this, which is why i linked all the lessons above, but you can continue to the next lesson by clicking at the next lesson at the bottom of the page.
Also, here is a compact 'cheat sheet' with some basics for various Z80 systems: https://www.chibiakumas.com/z80/CheatSheet.pdf
The ZX Spectrum consists of a Z80A @ 3.5 MHz CPU with either 16k, 48k or 128K of RAM. Most demos are targeted at the Spectrum 128 because it includes more memory, a shadow screen buffer and an AY soundchip. The different models have slightly different timings - this will cause issues if you are doing cycle-exact effects like multi-color.
Setting up your development platform for the ZX Spectrum is quite easy, first get the following tools:
- SjASMPlus (z00m's fork) - cross-platform open source assembler with nice macros for creating Binaries, TAP, TRD and SNA snapshot files out of the box, embedded Lua scripting and support for similar CPUs (Z80/R800/Z80N/i8080/LR35902).
- Pasmo - for the .TAP create. Available at : http://pasmo.speccy.org
- Emulator(s): I Found ZX Sping, FUSE, UnrealSpeccy and EightyOne to work best for my usecase. Most emulators can read TAP, SNA and TRD files out of the box.
Upon startup (when called from basic), the following values can assumed:
- The alternate HL register is set to 0x2758
- BC = start address
- A = C
Video display on the ZX Spectrum is mostly CPU based with little hardware features. No hardware sprites, no specific text or video modes, only a 256x192 byte screenbuffer with 1bit pixeldata located at $4000 in memory. It is ordened a bit strange in 3 sections of 256x64 pixels, then character rows, then subrows.
Address = 010RRLLL RRRCCCCC
- where RRRRR is the row number (0..23)
- CCCCC is the column number (0..31)
- LLL is the line number within the cell (0..7)
Calculating a screen address from XY coordinates is complicated due to the weird screen layout. In a larger demo you would generate a lookup table - it's usually best to avoid such calculations in small demos, but it can be done in under 30 bytes, eg: http://www.breakintoprogram.co.uk/computers/zx-spectrum/screen-memory-layout
The ZX Spectrum has a 32x24 colormap located at $5800 where you can write color information for each 8x8 tile. It has has 8 colors (INK and PAPER) with 2 brightness settings that can be set like this.
color = brightness(64) | (PAPER<<3) | INK
Because updating pixel memory can be slow, especially when you are grasping for bytes, some of the tiny intros on the zx spectrum prefer to use the colorram for the effect, sometimes in combination with an overlaying pattern.
You can set the border color to any of the 8 colors with:
ld a,0 ; bottom three bits of a contain the color out (254),a
128K separated to 8 pages (16384 bytes size) Default configuration is:
- page 5: $4000-$7fff
- page 2: $8000-$Bfff
- page 0: $C000-$Ffff
There are two screens - page 7 and page 5. port $7FFD allow to control memory and screens:
- bits 0-2 - page number mapped to memory at $C0000
- bit 3 - Select page 5(0) or page 7(1) to be displayed.
- bit 4 - ROM Select. 0-128K,1-48K
Example of use
loop: ei halt pg: ld a,$17 ld bc,$7ffd out (c),a xor $0A ld (pg+1),a ... do something with $C000-$DB00 jp loop
$17 is use 48K, map page 7 After xor $0A we'll get value $1D(use 48K ROM, display screen at page 7 and map page5 at $C000).This is so-called "double buffering". See also : https://worldofspectrum.org/faq/reference/128kreference.htm
Getting something on screen
Now to get something on screen, lets fill our colorram with a simple AND pattern, like so:
ld de,5800h ld b,24 yloop: ld c,32 xloop: ld a,c and b and 7 ; make sure range is 0..7 ld (de),a inc de dec c jr nz,xloop djnz yloop
Using a Backbuffer
While the above code will run fine, you might want to consider using a backbuffer for more complex stuff. You can then simply write to another adress define by BACKBUFFER (for example A000) and copy the buffer to colorram like so:
halt ; synchronize ld hl,BACKBUFFER ld de,5800h ld bc,768 ldir
Another alternative method is to use the 128's memory paging, which provides a second screen buffer. This buffer is located at a different memeory location, but otherwise it is the same:
; main loop starts here ld a,00010111b ; set up memory banks and screen here mainloop halt ; sync xor 00001010b ; flip screens out ($fd),a push af ; render code goes here ; screen buffer is location at 0xC000 instead of 0x4000 pop af jr mainloop
Overlaying simple graphics
If you don't want to use a solid color/tile for. You could copy a single tile across the screen at startup for some flair.
ld a,0x55 ; 01010101 pattern ld bc,0x1800 copyloop: ld (de),a inc de dec c jr nz,copyloop djnz copyloop
Alternatively you could generate a pattern using logic operations or random noise/data.
;calculate address of next line,HL=address down_hl: INC h LD A,h AND 7 RET NZ LD A,L ADD A,#20 LD L,A RET C LD A,H SUB 8 LD H,A RET
;move up at screen, HL=address up_hl LD A,H DEC H AND 7 ret nz LD A,L SUB 32 LD L,A ret c LD A,H ADD A,8 LD H,A ret
;write pixel CALL 8933 ; C=X(0..255),B=Y(0..175),5C7D=COORDS
;Calculate vertical line screen Address, IN : A=Y coordinate, OUT : HL=address py2saddr: ld l,a ; ld a,l and $07 ld h,a ld a,l and $c0 rra inc a rrca rrca or h ld h,a ld a,l add a add a and $e0 ld l,a
;Calculate Attribute address, IB: reg D=Y,reg E=X, OUT: HL=address, destroys DE ld l,d add hl,hl add hl,hl ld h,$11 add hl,hl add hl,hl add hl,hl ld d,0 add hl,de
or alter version
;H=Y coordinate,L=X coordinate ld a,h rrca rrca rrca push af and 3 add a,$58 ld h,a pop af and %11100000 add a,l ld l,a
calculate Pixel coordinate
;IN: A=X coordinate,C=L=Y coordinate ;OUT: HL=screen address,A=bit value call #22B0
The original Spectrum has only a 1 bit sound capability (BEEP) through its internal speaker. Later models included the AY-3-8910 Soundchip which provides 3 channels of PSG sound.
Make some noise - Beeper
Setting/toggling bit 4 of port 254 will enable the beeper speaker.
ld a,$10 ;Bit 4 set beeploop: xor $10 ;toggle Bit 4 out (254),a ld b,90 ;wait djnz $ jp beeploop
Note that the bottom three bits of port 254 also set the border color (see above).
Make some noise - AY
You can access the AY soundchip by outputting to the following ports:
ld bc,0xfffd ld a, ay register number out (c),a ld b,0xbf ld a, data byte out (c),a
For more information about the soundchip, check out: https://www.atarimagazines.com/v4n7/stsound.html
Small demos with documented source code
- Ceci N'est Pas Un Cube by Ate Bit https://www.pouet.net/prod.php?which=29691
- Zxwister by Ate Bit https://www.pouet.net/prod.php?which=44123
- Muse by Ate Bit https://www.pouet.net/prod.php?which=62880
- Starlet Guitarlet by HOOY-PROGRAM https://www.pouet.net/prod.php?which=52553
- Clangers On The Dancefloor by HOOY-PROGRAM https://www.pouet.net/prod.php?which=29696
- Chessington World Of Adventure by HOOY-PROGRAM https://www.pouet.net/prod.php?which=76074
- Roll Me Gently by Joker https://www.pouet.net/prod.php?which=86338
I found resources on ZX Spectrum sizecoding to be sparse.
- All kinds of Z80 Information http://www.z80.info/index.htm
- Another source for good Z80 Information http://z80-heaven.wikidot.com
- Assembler subforum on world of spectrum https://worldofspectrum.org/forums/categories/assembler
- Development subforum on Spectrum Computing https://spectrumcomputing.co.uk/forums/viewforum.php?f=6
- Blogpost on ZX Spectrum coding from a X86 coder's perspective on superogue's sizecdoing blog http://marquee.revival-studios.com/blog/blog_fluxus.html
- 128byte/256 byte zx spectrum productions by goblinish https://www.pouet.net/groups.php?which=11696&order=type
- 128byte/256 byte zx spectrum productions by gasman https://www.pouet.net/groups.php?which=1218&order=type
- Easy to read list of all Z80 instructions (including undocumented ones) with size and timing information https://clrhome.org/table/
- Detailed information on the screen layout http://www.overtakenbyevents.com/lets-talk-about-the-zx-specrum-screen-layout/ http://www.overtakenbyevents.com/lets-talk-about-the-zx-specrum-screen-layout-part-two/ and http://www.overtakenbyevents.com/lets-talk-about-the-zx-specrum-screen-layout-part-three/