90 lines
1.3 KiB
NASM
90 lines
1.3 KiB
NASM
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section .bss
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curpsp: resw 1
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absolute 0
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resb 2 ; ret 0 exit
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resb 2 ; allocation length
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resb 1
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resb 5 ; CP/M entry point
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; SS:SP, DS:DX, ES:BX and AX from the program
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PSPAX: resw 1
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PSPSS: resw 1
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PSPSP: resw 1
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PSPDS: resw 1
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PSPDX: resw 1
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PSPES: resw 1
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PSPBX: resd 1
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section .text
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; OUT ds PSP segment
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pspds: xor ax, ax
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mov ds, ax
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mov ds, [curpsp]
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ret
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int21: push ds
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; load program PSP and save userdata
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push ax
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call pspds
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pop word [PSPAX]
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pop word [PSPDS]
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mov [PSPSS], ss
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mov [PSPSP], sp
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mov [PSPDX], dx
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mov [PSPES], es
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mov [PSPBX], bx
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mov ss, ax
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mov sp, ( stack+stacksize )
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mov al, [PSPAX+1]
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shl ax, 1
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shl ax, 1
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call .etbl
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; syscall table
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; cells: ptr to handler, ptr to sysret
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dw restart, sret
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dw getc, sretb
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dw putc, sret
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; set up a return chain and execute it
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; first return into handler function
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; second return into appropiate sysret
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.etbl: pop bx
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add bx, ax
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push word [cs:bx+2]
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push word [cs:bx]
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ret
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; sysret handlers
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; return ES:BX to user
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sretd: call pspds
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jmp sret.l02
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; return BX to user
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sretw: call pspds
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mov es, [PSPES]
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jmp sret.l02
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; return AL to user
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sretb: push ax
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call pspds
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pop ax
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mov ah, [PSPAX+1]
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les bx, [PSPBX]
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jmp sret.l03
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; return without result
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sret: call pspds
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.l01: les bx, [PSPBX]
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.l02: mov ax, [PSPAX]
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.l03: mov ss, [PSPSS]
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mov sp, [PSPSP]
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lds dx, [PSPDX]
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iret
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