Z80 Assembly programming for the ZX Spectrum

When I was young, The ZX Spectrum was the cheapest of the 8 bits, and frequently looked down upon by CPC and C64 owners... Despite its more limited graphics, they do yield some interesting advantages... compared to the CPCs 4 color mode 1... the ZX Spectrum has similar resolution, and twice the onscreen colors - what's more, it uses half the screen memory which means Spectrum games are often significantly smoother than their CPC equivalents...

These days, clever developers are able to work around the spectrums color limitations, and produce impressive looking games with fast gameplay on this classic 8 bit.

Cpu	3.5mhz Z80	3.5mhz Z80
Ram	48k/128k	128k
Vram	8k	8k
Resolution 4-color	256x192 8 color / 2 brightness per 8x8 tile	256x192 8 color / 2 brightness per 8x8 tile
Sound chip	Beeper	AY-3-8910

Chibiakumas Tutorials:

	Lesson H2 - Hello World on the ZX Spectrum
	Lesson A2 - Interrupt Mode 2
	Lesson P1 - Basic Firmware Text functions
	Lesson P2 - More Text Functions, Improvements... and the Sam Coupe!
	Lesson P4 - Bitmap graphics on the ZX Spectrum and Sam Coupe
	Lesson P6 - Keyreading on the Amstrad CPC, ZX Spectrum and Sam Coupe
	Lesson P18 - Making Sound with the AY-3-8910 on the Amstrad CPC, MSX,ZX Spectrum.... and NeoGeo + Atari ST!!
	Lesson P23 - Sound with the 'Beeper' on the ZX Spectrum and Apple II
	Lesson P26 - Bankswitching and hardware detection on the ZX Spectrum
	Lesson P35 - Playing Digital Sound with WAV on the AY!
	Lesson P37 - Playing Digital Sound with WAV on the Sam Coupe, Camputers Lynx and ZX Spectrum

The Keyboard

We can read in a row of the keyboard by setting BC to the correct value and using IN A,(C)... the resulting byte will contain a 0 if the key is pressed, 1 if it is not.

	C= &FE
B=...	4	3	2	1	0
%11111110	V	C	X	Z	SHIFT
%11111101	G	F	D	S	A
%11111011	T	R	E	W	Q
%11110111	5	4	3	2	1
%11101111	6	7	8	9	0
%11011111	Y	U	I	O	P
%10111111	H	J	K	L	ENTER
%01111111	B	N	M	DEL	SPC

AY-3-8910 Sound Chip:

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)

For more details, please see the AY sound chip PDF

Beeper Sound Chip:

The "Beeper" sound chip is incredibly crude... it is controlled by bit 5 of the port &FE... by turning it on and off we can make simple sounds...

See the example to the right... by changing the pause (caused by BC) we can change the pitch of the sound... 3000 will be a relatively low pitch... 500 will be higher...

Some clever programs even manage to "Fake" multiple sound channels!

The big disadvantage to all this is that the CPU will be busy during the whole time, so the Beeper chip isn't very helpful, and we'll want to use the AY sound chip on the 128k systems... but on the 48k machines, it's all we've got!

    xor a
loopy:
    xor %00010000    ;---S-BBB        S=Sound B=Border
    out (&fe),a
    ld bc,3000    ;Lower number=higher pitch
pausey:
    dec c
    jr nz,pausey
    dec b
    jr nz,pausey
    jr loopy

Ram Banking
Ram banking is controlled by port &7FFD and &1FFD - they can be written, but not read, therefore, you should keep a backup of the value last sent to this port... by default the firmware keeps one at &5B5C and &5B67

Port	Backup	Bits	Details
&7FFD	&5B5C	- - IRSMMM	MMM= ram bank at C000 (0-7) S=Screen page bit R=Rom Low bit I=I/O Disabling
&1FFD	&5B67	- - - SDR - P	P = paging mode (0=normal 1=+3) R=Rom high bit D = Disk Motor S=Printer strobe

The ZX Spectrum 128 has 4 banks of 16k, the first is always rom on the 128k... the +3 CAN have ram in this bank, but this will mean you cannot support the 128k system (only about 15% of spectrums on the market are +3's)
Note... the Black +2 has the same hardware as the +3... the Grey +2 has the same hardware as the spectrum128 system

ZX 128K
&0000	ROM
&4000	Screen 1 (5)*
&8000	Ram (2)
&C000	*Screen 2 (7)*

* ZX Firmware uses &5B00-&6000

Ram Contention
'Contention' is banks of memory which are slower due to sharing with the screen memory, unfortunately, the banks that are contended are different on the 128k machines and the +3

128K	+3
Ram 0	Ram 0
Ram 1	Ram 1
Ram 3	Ram 3
Ram 4	Ram 4
Ram 6	Ram 6
Dark=Contended

Spectrum +3 Ram Options
As Mentioned, the spectrum +3 has some special banking options, which were used to allow CPM to work on the Spectrum - they are enabled by setting bit 0 of &1FFD to '1' to turn on this special mode

	&1FFD Bits 2,1
	00	01	10	11
&C000	Bank 3	Bank 7	Bank 3	Bank 3
&8000	Bank 2	Bank 6	Bank 6	Bank 6
&4000	Bank 1	Bank 5 (S)	Bank 5 (S)	Bank 7 (S)
&0000	Bank 0	Bank 4	Bank 4	Bank 4
	S=Screen Bank

Memory Map

48k		Usage
0000	3FFF	ROM
4000	57FF	Screen Ram
5800	5AFF	Screen Ram Color Data
5B00	5BFF	Printer Buffer (Sysvars on +3)
5C00	5CBF	System Vars
5CC0	5CCA	Reserved
5CCA	5D3B	TR-DOS Area
5D3B	FF57	Available Memory (Between Prog and Ramtop
FF58	FFFF	Reserved (User defined characters)

Screen Map

Calculating screen addresses for bitmap pixel data can be performed with the following 'formula'

H								L
7	6	5	4	3	2	1	0	7	6	5	4	3	2	1	0
0	1	0	Y7	Y6	Y2	Y1	Y0	Y5	Y4	Y3	X4	X3	X2	X1	X0

Calculating screen addresses for color block data can be performed with the following 'formula'

H								L
7	6	5	4	3	2	1	0	7	6	5	4	3	2	1	0
0	1	0	1	1	0	Y7	Y6	Y5	Y4	Y3	X4	X3	X2	X1	X0

A HL Pixel address (&4000-&57FF) can be converted to a color address (&5800-&5AFF) with the following commands:

ld a,h ;HL is a pixel address srl a srl a srl a add &50 ld h,a ;Address is now the equivalent color address

Each Color byte is defined in the following format:

%	7	6	5	4	3	2	1	0
	Flash	Bright	Back	Back	Back	Fore	Fore	Fore

FOREground and BACKground colors are defined by 3 bits each... values from 0-7:

0	1	2	3	4	5	6	7
Black #000000	Blue #0000FF	Red #FF0000	Magenta #FF00FF	Green #00FF00	Cyan #00FFFF	Yellow #FFFF00	White #FFFFFF

Spectrum +3 Disk File Header

Position	Bytes	Content	Details	Example
&0000	8	PLUS3DOS	Text Header	PLUS3DOS
&0009	1	EOF byte	EOF Character	26
&000A	1	Issue Num	Issue Num	1
&000B	1	Version Num	Version Num	0
&000F	4	Size+128	Size INC Header	&1080
&0010	2	Size	Size of file	&1000
&0012	5	Basic Header	Basic Header	&03,&00,&80,&00,&80
&0017	104	Unused	Unused	0 0 0 0�
&007F	1	Checksum	Checksum of Header bytes 0-126 (MOD 256)	?
&0080	Program Code

Spectrum Links
Fuse - My Spectrum emulator of choice!
Spectrum 128k and Spectrum 48K reference - Great summary of the hardware - provides much of the info you'll want for ZX dev
Basic Manual - You'll want to know at least enough basic to do calls and operate the computer
Spectrum Computing Forum - 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

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