Implement some PoC of reading from disk
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216
boot/kernel.asm
216
boot/kernel.asm
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@ -1,88 +1,78 @@
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cpu 8086
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org 0x7C00
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org 0
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jmp 0x07C0:init
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stack: equ 0x7C00
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framsz: equ 0x300
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%include "inc/bpb.asm"
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jmp init
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banner: db "rdos", 0xA, 0xD, 0
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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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; syscalls push a register set to stack
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; this is for accessing it
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rsdssi: equ 0x08
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rsesdi: equ 0x0C
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rsbp: equ 0x10
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rslpad: equ 0x12
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rscsip: equ 0x14
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rsflag: equ 0x18
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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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lpads: times 0x30 call entry
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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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entry: push bp
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; BX+0C: ES:DI
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push es
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push di
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; BX+08: DS:SI
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push ds
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push si
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; BX+00: AX,CX,DX,BX
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push bx
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push dx
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push cx
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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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mov ax, ss
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mov es, ax
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mov bx, 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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; This needs to be done when coming from userspace
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mov ax, cs
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mov ss, ax
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xor sp, sp
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int 3
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; Jump back to userspace
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ujump: cli
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mov ax, es
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mov ss, ax
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mov sp, bx
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; Pop whole registerset
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; base registers
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pop ax
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pop cx
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pop dx
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pop bx
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; string ptrs
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pop si
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pop ds
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pop di
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pop es
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; BP is special
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pop bp
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; skip landing pad number
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add sp, 2
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; pop IP, CS and flags
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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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; Processes register set DS:BX as syscall
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scall: ; Clear status flags
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and byte [bx+rsflag], 0
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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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; No such subfunction
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sferr: ; set carry
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or byte [bx+rsflag], 1
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ret
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; Subfunction table
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@ -146,34 +136,93 @@ setint: push es
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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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bpb: times bpb_len db 0
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drvnum: db 0
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align 4
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drvpos: dd 0
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buffer: times 512 db 0
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init: xor ax, ax
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; Select active drive
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; IN dl drive number A=0, B=1, C=2, ...
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select: mov [drvnum], dl
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ret
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; Set sector number
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seek: mov [drvpos], ax
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mov [drvpos+2], dx
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ret
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; Read sector from disk
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read: ; this uses the EBIOS extensions
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; qword sector number
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xor ax, ax
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push ax
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push ax
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mov ax, [drvpos+2]
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push ax
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mov ax, [drvpos]
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push ax
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; dword target buffer
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mov ax, buffer
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push cs
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push ax
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; word number of sectors
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mov ax, 1
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push ax
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; size & passing
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mov di, 0x10
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push di
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mov si, sp
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mov ah, 0x42
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mov dl, [drvnum]
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call dnconv
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stc
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int 0x13
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add sp, di
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ret
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; Convert between drive number and BIOS dl
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; Bidirectional mapping
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; 0 <-> 0, 1 <-> 1, 2 <-> 0x80, 3 <-> 0x81
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dnconv: mov cx, 7
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ror dx, 1
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rol dl, 1
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dncl: rcl dl, 1
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rcr dh, 1
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loop dncl
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xchg dh, dl
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ret
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init: cli
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mov ax, cs
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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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xor dh, dh
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call dnconv
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call select
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mov ax, 0x2520
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mov dx, int20h
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int 0x21
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xor ax, ax
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xor dx, dx
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call seek
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mov ax, 0x3520
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int 0x21
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call read
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mov ah, 0x3f
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int 3
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int 0x21
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mov ax, [buffer+0x1FE]
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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 si, banner
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mov ah, 0x0e
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xor bx, bx
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loop: lodsb
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@ -184,4 +233,3 @@ loop: lodsb
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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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