Implement whacky first version for 25h disk read
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177
boot/kernel.asm
177
boot/kernel.asm
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@ -4,6 +4,19 @@
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%include "inc/bpb.asm"
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; Far call via interrupt vector
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%macro intcall 1
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call far [cs:%1*4]
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%endmacro
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; Far jump via interrupt vector
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%macro intjump 1
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jump far [cs:%1*4]
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%endmacro
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; stack size
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%define stacks 512
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banner: db "rdos", 0xA, 0xD, '$'
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; Alias for Int 21h,AH=0h
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@ -26,7 +39,7 @@ int21h: ; inside of kernel, direction always goes up
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; do the actual subfunction call
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call [cs:sfptr]
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; inherit the lower 8 flag bits to userspace
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push ax
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iretfl: push ax
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push bp
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mov bp, sp
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lahf
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@ -239,7 +252,105 @@ ldbpb: push ds
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pop ds
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ret
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; DOS 2+ - DOS IDLE INTERRUPT
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; defines for CHS table
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chs_spt: equ 0 ; word sectors per track
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chs_nos: equ 2 ; word number of sides
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chs_po: equ 4 ; dword partition offset
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chs_siz: equ 8
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fndchs: ; find CHS table entry
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; IN dl DOS drive number (A=0, B=1, ...)
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; OUT cs:si ptr to chs table entry
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push ax
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mov ax, chs_siz
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mul dl
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add ax, chstab
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mov si, ax
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pop ax
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ret
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; compare sector number in disk access packet with zero
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; sets zero flag accordingly
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iszero: push ax
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mov ax, [bx]
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or ax, [bx+2]
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pop ax
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ret
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; calculate CHS from dap and CHS table entry
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; IN ds:bx disk access packet
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; cs:si chstab table entry
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; OUT cx,dh chs data for int 13h
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calchs: push ax
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; load linear sector number
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mov dx, [bx+2]
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mov ax, [bx]
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; if any word of it is non-zero, we need to do calculation
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call iszero
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jz .zero
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; ax is track number (lba / spt)
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; dx is sector (lba % spt) + 1
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div word [cs:si+chs_spt]
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inc dx
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; sector number
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mov cl, dl
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; ax is cylinder (track / heads)
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; dx is head (track % heads)
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xor dx, dx
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div word [cs:si+chs_nos]
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; set up cylinder and head number
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mov ch, al
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mov dh, dl
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pop ax
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ret
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; set up CHS data for int 13h for reading sector zero
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.zero: mov cx, 1 ; C=0, S=1
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mov dh, 0 ; H=0
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pop ax
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ret
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; ABSOLUTE DISK READ / DISK WRITE
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int25h: mov ah, 2
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jmp adisk
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int26h: mov ah, 3
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adisk: push bp
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push ds
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push es
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mov bp, sp
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cmp cx, 0xFFFF
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je .islrg
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; build a disk access packet on the stack
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push ds ; dword transfer buffer
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push bx
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push cx ; word number of sectors
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push cs ; dword starting sector number (CS always 0)
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push dx
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; set our freshly created dap as DS:BX
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mov bx, sp
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push ss
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pop ds
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.islrg: mov dl, al
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call fndchs
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call calchs
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mov dl, al
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mov al, [bx+4]
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mov es, [bx+8]
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mov bx, [bx+6]
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int 0x13
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jc .ret
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.ret: mov sp, bp
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pop es
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pop ds
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pop bp
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retf: retf
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; DOS IDLE INTERRUPT
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; Usually hooked by TSRs
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idle: sti
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; sti takes one instruction to take effect
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@ -248,7 +359,7 @@ idle: sti
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hlt
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iret
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; DOS 2+ - FAST CONSOLE OUTPUT
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; FAST CONSOLE OUTPUT
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; IN al character to print
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fputc: push ax
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push bx
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@ -259,36 +370,42 @@ fputc: push ax
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pop ax
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iret
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; write CS:AX to ES:DI and advance DI
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; used for setting interrupts
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stovec: stosw
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vects: dw int20h, int21h, retf, retf
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dw retf, int25h, int26h, int20h
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dw idle, fputc
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main: ; zero out first 64kb except our code
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; bss and boot sector area gets cleared by this
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mov di, init
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mov cx, di
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neg cx
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xor ax, ax
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rep movsb
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; install interrupt vectors
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mov si, vects
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mov di, (0x20*4)
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mov cx, 0x0A
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intlp: movsw
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mov ax, cs
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stosw
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ret
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vects: dw int20h, int21h, idle, fputc
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main: mov si, vects
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mov di, (0x20*4)
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lodsw
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call stovec
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lodsw
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call stovec
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mov di, (0x28*4)
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lodsw
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call stovec
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lodsw
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call stovec
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loop intlp
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; print banner to indicate we are booted
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mov si, banner
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mov ah, 9
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int 0x21
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call dnconv
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mov al, dl
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mov cx, 1
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mov dx, 0
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mov bx, buffer
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int 3
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intcall 0x25
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int 3
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loop: int 0x28
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mov ah, 7
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int 0x21
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jz loop
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int 0x29
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jmp loop
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init: cli
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@ -296,7 +413,7 @@ init: cli
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mov ds, ax
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mov es, ax
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mov ss, ax
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mov sp, stacke
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mov sp, ( stack+stacks )
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; relocate
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mov si, 0x7C00
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@ -307,11 +424,15 @@ init: cli
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jmp 0:main
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section .bss
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; stack to be used during init and disk i/o
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stack: resw stacks
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; default int 25h/26h handler use a cache to keep geometry for
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; the first 4 drives
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chstab: resw (4*chs_siz)
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bpb: resb bpb_len
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drvnum: resb 1
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align 4
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drvoff: resd 1; partition offset
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drvpos: resd 1; absolute physical sector number
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buffer: resb 512
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stack: resw 512
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stacke:
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