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Implement syscall subfunction jump table

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This commit is contained in:
Nero 2020-04-13 18:03:54 +02:00
parent 39ef1f9a3e
commit 6b49a20865
1 changed files with 62 additions and 15 deletions
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77
boot/kernel.asm
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@ -26,8 +26,8 @@ int21h: push bp
; Save user stack
mov ax, ss
mov [stack-2], ax
mov [stack-4], sp
mov [cs:stack-2], ax
mov [cs:stack-4], sp
; Set up kernel stack
xor ax, ax
@ -48,7 +48,7 @@ scall: sub sp, framsz
mov bp, sp
call getax
int 3
call jmptab
; Restore user stack
mov sp, [bp+framsz]
@ -59,6 +59,19 @@ scall: sub sp, framsz
pop bp
iret
; Lookup address of subfunction
; Mesh up the stack so we return to subfunction
; and subfunction later returns to scall
jmptab: push ax
push bx
xor bx, bx
add bl, ah
add bl, ah
mov bx, [cs:bx+sftab]
mov [bp-4], bx
pop bx
ret
; Get AX from the user stack
getax: push ds
push si
@ -73,40 +86,66 @@ sferr: stc
ret
; Subfunction table
align 2
sftab: dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
; 10
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
; 20
dw sferr, sferr, sferr, sferr
dw sferr, setint, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
; 30
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, getint, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
; DOS 25h: Set interrupt vector
; IN al interrupt number
; ds:dx entry point
setint: push ax
push bx
; IN al number
; OUT bx al * 4
times4: push ax
xor ah, ah
add al, al
add al, al
mov bx, ax
mov [cs:bx], dx
mov ax, ds
mov [cs:bx+2], ds
pop bx
pop ax
ret
; DOS 2+ - GET INTERRUPT VECTOR
; IN al interrupt number
; OUT es:bx current interrupt handler
getint: push ds
; DS=0
xor bx, bx
mov ds, bx
; BX=AL*4
call times4
les bx, [bx]
pop ds
ret
; DOS 25h: Set interrupt vector
; IN al interrupt number
; ds:dx entry point
setint: push es
push bx
; ES=0
xor bx, bx
mov es, bx
; BX=AL*4
call times4
mov [es:bx], dx
mov ax, ds
mov [es:bx+2], ds
pop bx
pop es
ret
; ===== end of resident, begin of transient startup code
init: xor ax, ax
@ -122,7 +161,15 @@ init: xor ax, ax
mov ax, 0x2520
mov dx, int20h
call setint
int 0x21
mov ax, 0x3520
int 0x21
mov ah, 0x3f
int 3
int 0x21
int 3
main: push cs
pop ds