STACK_TOP = 0xFFFF
DELAY_LOOPS_PER_MS = 40
RESIDENT_BASE = 0xC000
RESIDENT_ROM_SOURCE = 0x0080

        .include "bios.inc"

        .globl _main
        .globl _delay_ms
        .globl _io_out
        .globl _io_in
        .globl _enable_interrupts
        .globl _disable_interrupts
        .globl _jump_to
        .globl _bios_putc
        .globl _bios_try_getc
        .globl _bios_getc
        .globl _bios_init
        .globl resident_image_end
        .globl s__DATA
        .globl l__DATA
        .globl s__INITIALIZER
        .globl s__INITIALIZED
        .globl l__INITIALIZER

.area _BOOT (ABS)

.org 0x00
        jp boot_copy
.org 0x08
boot_copy:
        di
        ld sp, #STACK_TOP
        ld hl, #RESIDENT_ROM_SOURCE
        ld de, #RESIDENT_BASE
        ld bc, #(resident_image_end - RESIDENT_BASE)
        ldir
        jp init

.area _ITABLE (ABS)

; The executable vector page is part of the image copied to high RAM.
.org RESIDENT_BASE
        jp init
.org RESIDENT_BASE + 0x08
        reti
.org RESIDENT_BASE + 0x10
        reti
.org RESIDENT_BASE + 0x18
        reti
.org RESIDENT_BASE + 0x20
        reti
.org RESIDENT_BASE + 0x28
        reti
.org RESIDENT_BASE + 0x30
        reti
.org RESIDENT_BASE + 0x38
        reti

; IM 2 table: the CTC supplies vectors 40, 42, 44, and 46.
.org RESIDENT_BASE + 0x40
        .dw ctc_isr          ; CTC channel 0: vector 0x40
        .dw ctc_isr          ; CTC channel 1: vector 0x42
        .dw ctc_isr          ; CTC channel 2: vector 0x44
        .dw uart_isr         ; CTC channel 3: vector 0x46

; Stable BIOS jump table for ROM-resident applications.
; C050 putc(A), C053 try_getc(), C056 getc(), C059 io_out(A,L),
; C05C io_in(A), C05F delay_ms(HL), C062 monitor warm start.
.org BIOS_PUTC
        jp bios_service_putc
.org BIOS_TRY_GETC
        jp bios_service_try_getc
.org BIOS_GETC
        jp bios_service_getc
.org BIOS_IO_OUT
        jp bios_service_io_out
.org BIOS_IO_IN
        jp bios_service_io_in
.org BIOS_DELAY_MS
        jp bios_service_delay_ms
.org BIOS_MONITOR
        jp init

.area _HOME
init:
        di
        ld sp, #STACK_TOP

        call gsinit

        ; Setup interrupt mode 2
        ld a, #0xC0
        ld i, a
        im 2
        call _bios_init
        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:
        xor a                         ; BC loop test below changes A
        ld (hl), a
        inc hl
        dec bc
        ld a, b
        or c
        jr nz, clear_loop

gsinit_done:
.area _GSFINAL
        ret
resident_image_end:

.area _CODE

; BIOS ABI services preserve registers except documented return values.
bios_service_putc:
        push    af
        push    bc
        push    de
        push    hl
        push    ix
        push    iy
        call    _bios_putc
        pop     iy
        pop     ix
        pop     hl
        pop     de
        pop     bc
        pop     af
        ret

bios_service_try_getc:
        push    bc
        push    de
        push    hl
        push    ix
        push    iy
        call    _bios_try_getc
        pop     iy
        pop     ix
        pop     hl
        pop     de
        pop     bc
        ret

bios_service_getc:
        push    bc
        push    de
        push    hl
        push    ix
        push    iy
        call    _bios_getc
        pop     iy
        pop     ix
        pop     hl
        pop     de
        pop     bc
        ret

bios_service_io_out:
        push    af
        push    bc
        push    de
        push    hl
        push    ix
        push    iy
        call    _io_out
        pop     iy
        pop     ix
        pop     hl
        pop     de
        pop     bc
        pop     af
        ret

bios_service_io_in:
        push    bc
        push    de
        push    hl
        push    ix
        push    iy
        call    _io_in
        pop     iy
        pop     ix
        pop     hl
        pop     de
        pop     bc
        ret

bios_service_delay_ms:
        push    af
        push    bc
        push    de
        push    hl
        push    ix
        push    iy
        call    _delay_ms
        pop     iy
        pop     ix
        pop     hl
        pop     de
        pop     bc
        pop     af
        ret

.macro MAKE_ISR entry, handler
        .globl handler
entry:
        push    af
        push    bc
        push    de
        push    hl
        push    ix
        push    iy

        ex      af, af'
        push    af
        exx
        push    bc
        push    de
        push    hl

        call    handler

        pop     hl
        pop     de
        pop     bc
        exx
        pop     af
        ex      af, af'

        pop     iy
        pop     ix
        pop     hl
        pop     de
        pop     bc
        pop     af
        ei
        reti
.endm

MAKE_ISR ctc_isr, _ctc_interrupt
MAKE_ISR uart_isr, _uart_interrupt

_enable_interrupts::
        ei
        ret

_disable_interrupts::
        di
        ret

; 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

; Execute code at HL. Because this routine is entered with CALL, a RET in the
; target returns to the monitor immediately after jump_to().
_jump_to::
        jp      (hl)

; Approximate millisecond delay.
; Input: HL = delay in milliseconds.
_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