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megagw.asm
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megagw.asm
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; ----------------------------------------------------------------------------
;
; Software emulator of the 8080 CPU for the MEGA65, intended for CP/M or such.
; Please read comments throughout this source for more information.
;
; Copyright (C)2017,2024 LGB (Gábor Lénárt) <lgblgblgb@gmail.com>
;
; ----------------------------------------------------------------------------
;
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation; either version 3 of the License, or
; (at your option) any later version.
;
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
; GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program. If not, see <http://www.gnu.org/licenses/>.
;
; ----------------------------------------------------------------------------
.INCLUDE "mega65.inc"
.INCLUDE "emu.inc"
.INCLUDE "cpu.inc"
.INCLUDE "console.inc"
.INCLUDE "disk.inc"
GW_BUFFER_SIZE = $100
.BSS
gw_buffer: .RES GW_BUFFER_SIZE
.ASSERT gw_buffer + GW_BUFFER_SIZE < $8000, error, "gw_buffer must be in the low-memory area."
.CODE
.EXPORT megagw
.PROC megagw
LDA cpu_a
CMP #calls
BCS ret ; carry is already set for error
ASL A
TAX
JMP (jump_table,X)
ret:
RTS
jump_table:
.WORD shutdown ; func 0
.WORD get_host_buffer_addr ; func 1
.WORD get_info_str ; func 2 will use the CP/M dma!!!
.WORD hdos_trap ; func 3
calls = (* - jump_table) / 2
.ENDPROC
; Format of HDOS call from the viewpoint of CP/M app:
;
; Input AND output register registers:
; B = host CPU register A
; C = host CPU register X
; D = host CPU register Y
; E = host CPU register Z
; Output:
; FLAGS = carry SET -> OK, carry CLEAR -> ERROR
; Input register:
; A = call ID, 0 -> HDOS trap
; HL = mem address of 256 bytes buffer!
; Data buffer must be appropiate size for the given call
;
; The call itself must be: CALL $FFFF
.PROC call_trap
STA trap_lo_byte
LDA cpu_l
STA copy1
STA copy2
LDA cpu_h
STA copy1+1
STA copy2+2
; Copy CP/M buffer for us
STA $D707 ; trigger in-line DMA
.BYTE $A,0 ; enhanced mode opts
.BYTE 0 ; DMA command: copy and not chained
.WORD GW_BUFFER_SIZE ; DMA length
copy1: .WORD 0 ; source addr, in case of "fill" the low byte is the byte to fill with
.BYTE I8080_BANK ; source bank + other info
.WORD gw_buffer ; target addr
.BYTE 0 ; target bank + other info
.WORD 0 ; modulo: not used
; Load 8080 registers into naitve registers for the TRAP
LDA cpu_b
LDX cpu_c
LDY cpu_d
LDZ cpu_e
; The TRAP itself
trap_lo_byte = * + 1
STA $D640 ; The trap!
CLV ; MEGA65 safety measure
; Save resulting native registers to 8080 registers
STA cpu_b
STX cpu_c
STY cpu_d
STZ cpu_e
; Check the carry flag, and transfer it to the 8080 carry flag
; (this also means that carry shouldn't be bothered between this point - ADC - and the trap itself above!!)
LDA cpu_f
AND #$FE ; clear 8080 carry flag (bit 0 will be cleared)
ADC #0 ; if host carry was set, this actually adds 1 (bit 0 will be set), so we're good :D
STA cpu_f
; Copy our buffer to CP/M buffer
STA $D707 ; trigger in-line DMA
.BYTE $A,0 ; enhanced mode opts
.BYTE 0 ; DMA command: copy and not chained
.WORD GW_BUFFER_SIZE ; DMA length
.WORD gw_buffer ; source addr, in case of "fill" the low byte is the byte to fill with
.BYTE 0 ; source bank + other info
copy2: .WORD 0 ; target addr
.BYTE I8080_BANK ; target bank + other info
.WORD 0 ; modulo: not used
; ... and we're done :D
CLC ; Valid call signal (nothing to do with the carry result of the trap itself!!!)
RTS
.ENDPROC
.PROC get_host_buffer_addr
LDA #.LOBYTE(gw_buffer)
STA cpu_l
LDA #.HIBYTE(gw_buffer)
STA cpu_h
CLC
RTS
.ENDPROC
.PROC hdos_trap
LDA #$40 ; trap addr low byte
JMP call_trap
.ENDPROC
.PROC shutdown
SEI ; Disable interrupts
LDA #$FF
STA 1
STA $D02F ; VIC KEY register, let's mess it up
JMP ($FFFC)
.ENDPROC
.PROC get_info_str
.IMPORT build_info_str
LDX #0
LDZ #0
: LDA build_info_str,X
STA32Z cpm_dma
BEQ :+
INX
INZ
BRA :-
: CLC
RTS
.ENDPROC