121 lines
2.1 KiB
NASM
121 lines
2.1 KiB
NASM
STACK_TOP = 0xFFFF
|
|
; At 500 kHz the CPU has about 500 T-states per millisecond.
|
|
; The inner delay loop below costs about 26 T-states per taken iteration:
|
|
; dec hl = 6T
|
|
; ld a,h = 4T
|
|
; or l = 4T
|
|
; jr nz = 12T
|
|
; 500 / 26 = 19.2, so 20 iterations is the nearest simple whole-number choice.
|
|
DELAY_LOOPS_PER_MS = 20
|
|
|
|
.globl _main
|
|
.globl _delay_ms
|
|
.globl _io_out
|
|
.globl _io_in
|
|
.globl s__DATA
|
|
.globl l__DATA
|
|
.globl s__INITIALIZER
|
|
.globl s__INITIALIZED
|
|
.globl l__INITIALIZER
|
|
|
|
.area _ITABLE (ABS)
|
|
|
|
.org 0x00
|
|
jp init
|
|
.org 0x08
|
|
reti
|
|
.org 0x10
|
|
reti
|
|
.org 0x18
|
|
reti
|
|
.org 0x20
|
|
reti
|
|
.org 0x28
|
|
reti
|
|
.org 0x30
|
|
reti
|
|
.org 0x38
|
|
reti
|
|
|
|
.area _HOME
|
|
init:
|
|
di
|
|
ld sp, #STACK_TOP
|
|
|
|
call gsinit
|
|
call _main
|
|
|
|
hang:
|
|
jr hang
|
|
|
|
.area _GSINIT
|
|
gsinit:
|
|
ld bc, #l__INITIALIZER
|
|
ld a, b
|
|
or c
|
|
jr z, zero_data
|
|
|
|
ld de, #s__INITIALIZED
|
|
ld hl, #s__INITIALIZER
|
|
ldir
|
|
|
|
zero_data:
|
|
ld bc, #l__DATA
|
|
ld a, b
|
|
or c
|
|
jr z, gsinit_done
|
|
|
|
ld hl, #s__DATA
|
|
xor a
|
|
|
|
clear_loop:
|
|
ld (hl), a
|
|
inc hl
|
|
dec bc
|
|
ld a, b
|
|
or c
|
|
jr nz, clear_loop
|
|
|
|
gsinit_done:
|
|
.area _GSFINAL
|
|
ret
|
|
|
|
.area _CODE
|
|
; Write a byte to an I/O port.
|
|
; Input: A = port address, L = data byte.
|
|
_io_out::
|
|
ld c, a
|
|
ld a, l
|
|
out (c), a
|
|
ret
|
|
|
|
; Read a byte from an I/O port.
|
|
; Input: A = port address.
|
|
; Return: A = data byte.
|
|
_io_in::
|
|
ld c, a
|
|
in a, (c)
|
|
ret
|
|
|
|
; Approximate millisecond delay.
|
|
; Input: HL = delay in milliseconds.
|
|
; For CLK_SPEED = 500000 Hz this uses 20 inner iterations per ms.
|
|
_delay_ms::
|
|
ld a, h
|
|
or l
|
|
ret z
|
|
delay_ms_outer$:
|
|
push hl
|
|
ld hl, #DELAY_LOOPS_PER_MS
|
|
delay_ms_inner$:
|
|
dec hl
|
|
ld a, h
|
|
or l
|
|
jr nz, delay_ms_inner$
|
|
pop hl
|
|
dec hl
|
|
ld a, h
|
|
or l
|
|
jr nz, delay_ms_outer$
|
|
ret
|