shadowrun96 / z80-notes

What is says on the tin.

Amstrad Z80 machine code notes

ORG &9c40

LD IX,&9C70
LD BC,&FFFE

LD (IX+0), B
LD (IX+1), C
RET	

3. How to add 2 16-bit numbers together?

ORG &9c40

LD IX,&9C70
LD HL,0
LD BC, 1
LD DE, 2

ADD HL,BC
ADD HL,DE

LD (IX+0),H
LD (IX+1),L
RET

or skip BC and just use two registers storing the value to HL.

LD Hl,1
LD DE,2

Do a 'for loop' from 0 to 10. We increment a memory address with a value of 0, ten times.

org &9c40

ld a,10
ld b,0
ld hl,&9ca0

loop:

	cp b
	jp z,loop_exit

	inc (hl)
	inc b
	jp loop	
loop_exit:
  ret

do a for loop, printing 0123456789 to the screen.

org &9c40

ld a,47        ; 47 is one character before '0'. 
ld hl,&9ca0    ; we load HL with the address to put '47' 
ld (hl),a      ; we load the address pointed at HL with 47


ld a,10        ; loop 10 times
ld b,0


loop:

	cp b
	jp z,loop_exit

	inc (hl)
	ld a,(hl)
	call &bb5a

	ld a,10
	inc b
	jp loop	
loop_exit:
	ret

Print '=' if two integers are equal, '<' if A is less than compared, '>' if greater

org #9c40

PrintChar equ #bb5a

ld a,10
sub 100

jp Z, equal
jp NC, greaterthan
jp C, lessthan
ret


lessthan: 
	ld a,'<'
	call PrintChar
	ret

greaterthan:
	ld a,'>'
	call PrintChar
	ret

equal:
	ld a,'='
	call PrintChar
	ret

Store 8-bit and 16-bit values to memory addresses.

org #9c40

; store an 8-bit value to a memory address
ld a,254
ld (#9c60), a

; store a 16-bit value to a memory address.
; This will get stored backwards, 'aa' in 9c70 and 'ff' in 9c71.

ld hl, #ffaa
ld (#9c70), hl

; store a 16-bit value from memory to a 16-bit register. This will also store the values
; in 9c70 and 9c71.

ld bc, (#9c70)

BF

Note The Data Pointer will probably be HL, since we need to be able to point it to a next cell, which is a 16-bit memory address. We can also increment the byte at the data pointer, so since we can only do 255 (ASCII) the MSB will be &00. Testing for it to be zero, will be tricky, since we need to artificially enforce memory boundaries, i.e. zero will not mean that HL is in address &0000, but in the ORG one. Incrementing the data pointer means ORG+1. We also need to make sure that the data pointer will not fall off the boundary (the ORG memory address).

org &0080

ld hl, &0080 ; initialise data pointer
inc hl		; > Increment the data pointer by one (to point to the next cell to the right).
dec hl		; < Decrement the data pointer by one (to point to the next cell to the left).
inc (hl)	; + Increment the byte at the data pointer by one.
dec (hl)	; - Decrement the byte at the data pointer by one.

ld a,(hl)
call &bb5a	; . output the byte at the data pointer

call &bb09
ld hl, a	; , Accept one byte of input, storing its value in the byte at the data pointer.
 
; test

jp brack1
jp brack2

brack1:

ret

brack2:

	; commands

; test if data pointer is zero (if HL=&0080)

ret 

sample loop1

ORG &0080
LD A,0 ; Accumulator is data pointer

; ++++++++++++++++++++++++++++++++++++++++++++++++++
; [.-]

PRINTCHAR EQU &BB5A

INC A
INC A
INC A
INC A
INC A
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INC A
INC A

CP 0
JP NZ,loop1
RET


loop1:
	CALL PRINTCHAR
	
	DEC A

	CP 0
	JP NZ,loop1
	ret

sample inner loop

ORG &0080
LD A,0 ; Accumulator is data pointer

; ++++++++++++++++++++++++++++++++++++++++++++++++++
; [.[-]]

PRINTCHAR EQU &BB5A

INC A
INC A
INC A
INC A
INC A
INC A
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INC A
INC A

CP 0
JP NZ,loop1
RET


loop1:
	CALL PRINTCHAR
	CP 0
	JP NZ, loop2
	ret

loop2:

	DEC A
	CP 0
	JP NZ,loop2
	ret

Draw a rectangle using the Amstrad Firmware

org &9c40
; call &bb4e
; mode 1, cursor at top left, no clear

call &bb6c ; CLS

; move absolute
ld hl, 200
ld de, 200
call &bbc0

ld hl, 200 ; x 
ld de, 400 ; y
call &bbf6

ld hl, 200 ; x 
ld de, 400 ; y
call &bbf6

ld hl, 100
ld de, 400
call &bbf6

ld hl, 100
ld de, 200
call &bbf6

ld hl, 200
ld de, 200
call &bbf6

ret

get/set cursor location example

ORG &9C40

;  Firmware calls we need. 

txt_output   EQU &BB5A 
km_wait_char EQU &BB06
txt_cur_on   EQU &BB81
txt_cur_enable EQU &BB7B
txt_get_cursor EQU &BB78
txt_set_cursor EQU &BB75
txt_set_column EQU &BB6F

CALL txt_cur_enable
CALL txt_cur_on
CALL txt_get_cursor

LD A,'X'
CALL txt_output
LD A,'X'
CALL txt_output
LD A,'X'
CALL txt_output

CALL txt_get_cursor
DEC H
CALL txt_set_cursor
LD A,&10
CALL txt_output

NOP
NOP

loop: jp loop

Text buffer using the Amstrad firmware ...

ORG &9C40

;  Firmware calls we need. 

LD HL,&9D00
CALL &BD5E
NOP

VRAM test

; flood vram with ff, let's see how that'll work.

scr_dot_position equ &BC1D 
org &1000

call scr_dot_position

ld (hl),255
inc hl
ld (hl), 255
inc hl
ld (hl), 255
inc hl
ld (hl), 255
inc hl
ld (hl), 255
inc hl
ld (hl), 255
inc hl
ld (hl), 255
ret

Split the MSB/LSB of an 8-bit number into two memory locations. For example '3D' becomes '30 0D'

org &4000
ld a, &3d
ld b, a
and %11110000
ld (&5001), a
ld a,b
and %00001111
ld (&5002), a
ret