rdos/boot/kernel.asm

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cpu 8086
org 0x500
%include "inc/bpb.asm"
%include "inc/mbr.asm"
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%include "inc/dpt.asm"
; kernel stack size in words
%define stacksize 512
init: cli
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov sp, ( stack+stacksize )
; relocate
mov si, 0x7C00
mov di, $$
mov cx, (_end-$$)
rep movsb
jmp 0:main
main: ; zero out first 64kb except our code
; bss and boot sector area gets cleared by this
mov cx, di
neg cx
xor ax, ax
rep stosb
; install interrupt vectors
mov si, vects
mov di, (0x20*4)
mov cx, 0x0A
intlp: movsw
mov ax, cs
stosw
loop intlp
; file descriptor 0 = CON
mov word [fd_table], con_ftab
; print banner
mov dx, banner
mov cx, 13
mov bx, 0
call fd_write
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call drives_init
loop: int 0x28
jmp loop
section .data
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banner: db "rdos loaded", 0xA, 0xD, '$'
section .text
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; Alias for Int 21h,AH=0h
int20h: xor ah, ah
jmp far [cs:0x21*4]
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int21h: ; inside of kernel, direction always goes up
; the iret will restore it to the user value later
cld
; set sfptr from ah
push bx
xor bx, bx
mov bl, ah
add bl, bl
add bx, sftab
mov bx, [cs:bx]
mov [cs:sfptr], bx
pop bx
; do the actual subfunction call
call [cs:sfptr]
; inherit the lower 8 flag bits to userspace
iretfl: push ax
push bp
mov bp, sp
lahf
mov [bp+8], ah
pop bp
pop ax
; iret frame: IP CS FLAGS
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iret: iret
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retf: retf
section .data
; Subfunction ptr
; this is used as extra register in int21h
sfptr: dw 0
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; Subfunction table
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sftab: dw sferr, getc, putc, sferr
dw sferr, sferr, conout, conin
dw sferr, puts, sferr, sferr
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dw sferr, sferr, sferr, sferr
; 10
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dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
; 20
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dw sferr, sferr, sferr, sferr
dw sferr, setint, sferr, sferr
dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
; 30
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dw sferr, sferr, sferr, sferr
dw sferr, getint, sferr, sferr
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dw sferr, sferr, sferr, sferr
dw sferr, sferr, sferr, sferr
section .text
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; OUT al character read
getc: xor ax, ax
int 0x16
push ax
jmp putc.2
; IN dl character to write
putc: push ax
mov al, dl
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.2: int 0x29
pop ax
ret
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; console output
; IN dl character
conout: cmp dl, 0xFF
je conine
push ax
mov al, dl
int 0x29
pop ax
ret
; console input with echo
; OUT al character
; zf clear when character available
conine: mov ah, 1
int 0x16
jnz .has
xor al, al
ret
.has: xor ax, ax
int 0x16
test al, al
jz conine
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int 0x29
test ax, ax
ret
; console input without echo
; OUT al character
; zf clear when character available
conin: mov ah, 1
int 0x16
jnz .has
xor al, al
ret
.has: xor ax, ax
int 0x16
test al, al
jz conin
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test ax, ax
ret
; DOS 1+ 9h - WRITE STRING TO STANDARD OUTPUT
; IN ds:dx '$'-terminated string
puts: push si
.loop: lodsb
cmp al, '$'
je .end
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int 0x29
jmp .loop
.end: pop si
ret
; DOS 1+ 25h - SET INTERRUPT VECTOR
; IN al interrupt number
; ds:dx entry point
setint: push bx
xor bx, bx
; BX=AL*4
mov bl, al
add bl, bl
add bl, bl
; save DS:DX into vector
mov [cs:bx], dx
mov [cs:bx+2], ds
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pop bx
ret
; DOS 2+ 35h - GET INTERRUPT VECTOR
; IN al interrupt number
; OUT es:bx current interrupt handler
getint: xor bx, bx
; BX=AL*4
mov bl, al
add bl, bl
add bl, bl
; load vector into ES:BX
les bx, [cs:bx]
ret
; Fallback for non-existant subfunctions
; The carry flag is inherited to user
sferr: stc
ret
; DOS IDLE INTERRUPT
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; Usually hooked by TSRs
idle: sti
; sti takes one instruction to take effect
nop
; Wait until next interrupt
hlt
iret
; FAST CONSOLE OUTPUT
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; IN al character to print
fputc: push ax
push bx
mov ah, 0x0E
xor bx, bx
int 0x10
pop bx
pop ax
iret
%include "kernel/fd.asm"
%include "kernel/con.asm"
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%include "kernel/drive.asm"
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%include "kernel/diskio.asm"
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%include "kernel/fat.asm"
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zero: dw 0
section .data
vects: dw int20h, int21h, retf, retf
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dw retf, retf, retf, int20h
dw idle, fputc
_end: dw 0x55AA, 0x55AA
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section .bss
; stack to be used during init and disk i/o
stack: resw stacksize