188 lines
2.9 KiB
NASM
188 lines
2.9 KiB
NASM
cpu 8086
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org 0x7C00
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stack: equ 0x7C00
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framsz: equ 0x300
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jmp init
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; Data on user stack while in kernel (5 words):
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; AX BP IP CS flags
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; Kernel stack:
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; [frame] SP SS
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; BP-^
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; The start of the frame is available via BP.
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; The frame is followed by a long ptr to the user stack.
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; Main entry point
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int20h: xor ah, ah
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int21h: push bp
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push ax
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; Test if SS=0, if it is, we assume we are already on kernel stack
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mov ax, ss
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test ax, ax
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jz rentry
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; Save user stack
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mov ax, ss
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mov [cs:stack-2], ax
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mov [cs:stack-4], sp
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; Set up kernel stack
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xor ax, ax
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mov ss, ax
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mov sp, (stack-4)
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jmp scall
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; This is like the above, except that it doesnt switch stacks.
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; The 'user' stack data is after (below on stack) the kernel frame.
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; [frame] SP SS AX BP IP CS flags
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; \-\--^
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rentry: mov ax, sp
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push ss
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push ax
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scall: sub sp, framsz
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mov bp, sp
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call getax
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call jmptab
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; Restore user stack
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mov sp, [bp+framsz]
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mov ax, [bp+framsz+2]
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mov ss, ax
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pop ax
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pop bp
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iret
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; Lookup address of subfunction
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; Mesh up the stack so we return to subfunction
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; and subfunction later returns to scall
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jmptab: push ax
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push bx
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xor bx, bx
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add bl, ah
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add bl, ah
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mov bx, [cs:bx+sftab]
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mov [bp-4], bx
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pop bx
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ret
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; Get AX from the user stack
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getax: push ds
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push si
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lds si, [bp+framsz]
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lodsw
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pop ds
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pop si
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ret
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; No such subfunction - set carry and exit
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sferr: stc
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ret
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; Subfunction table
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sftab: dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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; 10
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dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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; 20
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dw sferr, sferr, sferr, sferr
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dw sferr, setint, sferr, sferr
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dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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; 30
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dw sferr, sferr, sferr, sferr
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dw sferr, getint, sferr, sferr
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dw sferr, sferr, sferr, sferr
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dw sferr, sferr, sferr, sferr
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; IN al number
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; OUT bx al * 4
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times4: push ax
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xor ah, ah
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add al, al
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add al, al
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mov bx, ax
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pop ax
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ret
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; DOS 2+ - GET INTERRUPT VECTOR
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; IN al interrupt number
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; OUT es:bx current interrupt handler
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getint: push ds
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; DS=0
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xor bx, bx
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mov ds, bx
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; BX=AL*4
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call times4
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les bx, [bx]
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pop ds
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ret
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; DOS 25h: Set interrupt vector
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; IN al interrupt number
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; ds:dx entry point
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setint: push es
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push bx
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; ES=0
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xor bx, bx
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mov es, bx
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; BX=AL*4
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call times4
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mov [es:bx], dx
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mov ax, ds
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mov [es:bx+2], ds
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pop bx
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pop es
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ret
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; ===== end of resident, begin of transient startup code
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init: xor ax, ax
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mov ds, ax
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mov es, ax
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mov ax, 0x1000
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mov ss, ax
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xor sp, sp
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mov al, 0x21
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mov dx, int21h
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call setint
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mov ax, 0x2520
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mov dx, int20h
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int 0x21
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mov ax, 0x3520
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int 0x21
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mov ah, 0x3f
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int 3
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int 0x21
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int 3
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main: push cs
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pop ds
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mov si, msg
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mov ah, 0x0e
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xor bx, bx
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loop: lodsb
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test al, al
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jz end
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int 0x10
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jmp loop
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end: hlt
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jmp end
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msg: db "rdos kernel stub", 0x0A, 0x0D, 0
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