551 lines
7.7 KiB
NASM
551 lines
7.7 KiB
NASM
; SPDX-License-Identifier: MIT
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; Copyright (c) 2021 Juhani 'nortti' Krekelä.
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iosegment equ 60h
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dossegment equ iosegment + 1*1024/16 ; DOS starts 1KiB after IO system
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dos segment at dossegment
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org 0
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dosinit proc far
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dosinit endp
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dos ends
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code segment
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code ends
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constants segment
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constants ends
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data segment
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data ends
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iogroup group code, constants, data
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code segment
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assume cs:iogroup, ds:iogroup, es:iogroup
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org 0
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; Jump table
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jmp init ; 0000
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jmp near ptr status ; 0003
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jmp near ptr getch ; 0006
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jmp near ptr putch ; 0009
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jmp unimplemented ; 000c Output to printer
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jmp unimplemented ; 000f Serial read
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jmp unimplemented ; 0012 Serial write
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jmp near ptr diskread ; 0015
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jmp diskwrite ; 0018
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jmp near ptr diskchange ; 001b
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jmp setdate ; 001e
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jmp settime ; 0021
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jmp gettime ; 0024
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jmp near ptr flush ; 0027
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jmp near ptr mapdev ; 002a
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init:
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cld
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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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; Put setup stack just below 32K
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xor ax, ax
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mov ss, ax
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mov sp, 8000h
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sti
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; Figure out memory size
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int 12h
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; AX = memory size in kilobytes
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; We want it in paragraphs
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; There are 64 paragraphs in a kilobyte
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mov cx, 6
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shl ax, cl
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; Memory size is passed in dx
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mov dx, ax
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call hexprint16
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call newline
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; Disk table is passed in si
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mov si, offset iogroup:disks_table
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call dosinit
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; Memory for command.com is at ds, with PSP already set up
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; Retrieve amount of memory in the program segment
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mov cx, word ptr ds:6
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; Set up DTA
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mov dx, 100h
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mov ah, 1ah
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int 21h
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; Save command.com's segment for later and revert ds for opening the FCB
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mov bx, ds
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mov ax, cs
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mov ds, ax
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; Open command.com
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mov dx, offset iogroup:command_fcb
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mov ah, 0fh
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int 21h
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test al, al
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jnz open_error
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mov al, '+'
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mov ah, 0eh
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int 10h
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; Set random record field 0
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mov word ptr command_fcb+33, 0
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mov word ptr command_fcb+35, 0
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; Set record size to 1 byte
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mov word ptr command_fcb+14, 1
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; Read command.com into memory
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mov ah, 27h
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int 21h
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jcxz read_error
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cmp al, 1
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jne read_error
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mov ax, cx
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call hexprint16
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mov al, '.'
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mov ah, 0eh
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int 10h
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; Set up segments for command.com
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mov ds, bx
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mov es, bx
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mov ss, bx
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mov sp, 5ch ; Microsoft's documentation says to use this
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; Put a zero at top of the stack
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xor ax, ax
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push ax
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; Set default DTA
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mov dx, 80h
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mov ah, 1ah
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int 21h
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; Jump to command.com
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push bx
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mov ax, 100h
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push ax
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trampoline proc far
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ret
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trampoline endp
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; TODO: Better error reporting
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open_error:
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mov al, '-'
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jmp unfinished
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read_error:
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mov al, '!'
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unfinished:
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mov ah, 0eh
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int 10h
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jmp hang
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; OUT:
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; al = character if any
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; zf = there were no characters
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status proc far
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push bx
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push ax
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mov ah, 1
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int 16h
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mov bl, al
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pop ax
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mov al, bl
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pop bx
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ret
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status endp
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; OUT:
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; al = character
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getch proc far
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push bx
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push ax
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xor ah, ah
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int 16h
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mov bl, al
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pop ax
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mov al, bl
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pop bx
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ret
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getch endp
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; IN:
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; al = character
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putch proc far
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push ax
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mov ah, 0eh
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int 10h
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pop ax
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ret
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putch endp
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; IN:
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; al = driver number
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; ds:bx = buffer
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; cx = number of sectors to read
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; dx = LBA of first sector
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; OUT:
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; TODO: Document
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diskread proc far
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push ax
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mov ax, 0e00h + 'r'
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int 10h
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pop ax
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call far_caller
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push ax
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call hexprint8
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call space
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mov ax, ds
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call hexprint16
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call space
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mov ax, bx
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call hexprint16
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call space
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mov ax, cx
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call hexprint16
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call space
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mov ax, dx
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call hexprint16
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call newline
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pop ax
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; TODO: Everything except sregs can be trashed
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push es
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push ax
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push bx
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push cx
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push dx
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push di
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; Save driver number
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push ax
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; BIOS uses es:bx instead of ds:bx
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mov ax, ds
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mov es, ax
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mov ax, dx
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; Put drive(r) number in dl
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pop dx
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sector_read_loop:
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jcxz ret_diskread
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mov di, 3 + 1 ; 3 retries, + 1 since we dec first
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try_sector_read:
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push ax
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push cx
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call chs
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mov ah, 2
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mov al, 1
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int 13h
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jnc sector_read_success
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dec di
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jz sector_read_fail
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pop cx
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pop ax
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jmp try_sector_read
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sector_read_fail:
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mov al, ah
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call hexprint8;debg
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mov al, '?'
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mov ah, 0eh
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int 10h
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pop cx
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pop ax
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pop di
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pop dx
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pop ax ; Would be cx, don't overwrite
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pop bx
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pop ax
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pop es
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mov al, 12 ; TODO: Don't hardcode
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stc
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ret
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sector_read_success:
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pop cx
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pop ax
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dec cx
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inc ax
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add bx, 512
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jmp sector_read_loop
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ret_diskread:
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pop di
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pop dx
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pop cx
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pop bx
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pop ax
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pop es
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clc
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ret
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diskread endp
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; IN:
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; ax = LBA
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; OUT:
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; ch = cylinder
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; cl = sector & 2 high bits of cylinder
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; dh = head
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chs proc
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push ax
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push dx
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xor dx, dx
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; cylinder (track) - head - sector
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; cylinder = LBA / sectorspertrack / heads
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; head = LBA / sectorspertrack % heads
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; sector = LBA % sectorspertrack + 1
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div cs:sectorspertrack
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; ax = LBA / sectorspertrack
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; dx = LBA % sectorspertrack
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; sector
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mov cl, dl
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inc cl
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xor dx, dx
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div cs:heads
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; ax = LBA / sectorspertrack / heads
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; dx = LBA / sectorspertrack % heads
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; head
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mov dh, dl
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; cylinder (track)
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mov ch, al
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shr ax, 1
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shr ax, 1
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and al, 0c0h
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or cl, al
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mov ax, dx
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pop dx
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mov dh, ah
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pop ax
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ret
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chs endp
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diskwrite:
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mov al, 'w'
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jmp error
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; IN:
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; al = drive
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; OUT:
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; ah = -1 different / 0 dunno / 1 same
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; cf = 0 -> al = driver num
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; cf = 1 -> al = disk error code
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diskchange proc far
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; TODO: Implement
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mov ax, 0000h
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clc
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ret
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diskchange endp
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setdate:
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mov al, 'd'
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jmp error
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settime:
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mov al, 't'
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jmp error
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gettime:
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mov al, 'T'
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jmp error
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flush proc far
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push ax
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flush_loop:
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mov ah, 1
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int 16h
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jz flush_ret
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xor ah, ah
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int 16h
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jmp flush_loop
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flush_ret:
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pop ax
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ret
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flush endp
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mapdev proc far
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; TODO: Implement
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ret
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mapdev endp
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unimplemented:
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mov al, '@'
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error:
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mov ah, 0eh
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int 10h
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mov ah, 0eh
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mov al, '!'
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int 10h
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call far_caller
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hang:
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hlt
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jmp hang
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code ends
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; TODO: STRUC?
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constants segment
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sectorspertrack dw 8 ; TODO: Don't hardcode
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heads dw 2 ; TODO: Don't hardcode
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disks_table:
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db 1 ; 1 drive, TODO: Don't hardcode
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db 0 ; Physical drive 0
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dw offset iogroup:parameters_320k
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parameters_320k:
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dw 512 ; Sector size in bytes
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db 2 ; Sectors per cluster
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dw 1 ; Number of reserved sectors
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db 2 ; Number of FATs
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dw 112 ; Number of directory entries
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dw 320*2 ; Number of sectors
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constants ends
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data segment
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command_fcb:
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db 1 ; First drive, TODO: Don't hardcode
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db "COMMAND COM"
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db 25 dup (?)
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data ends
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code segment
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hexprint16 proc
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xchg al, ah
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call hexprint8
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xchg al, ah
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hexprint8 proc
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rol al, 1
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rol al, 1
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rol al, 1
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rol al, 1
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call hexprint4
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rol al, 1
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rol al, 1
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rol al, 1
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rol al, 1
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hexprint4 proc
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push ax
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and al, 0fh
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cmp al, 9
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jbe under_10
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add al, 'a' - 10 - '0'
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under_10:
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add al, '0'
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mov ah, 0eh
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int 10h
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pop ax
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ret
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hexprint4 endp
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hexprint8 endp
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hexprint16 endp
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newline proc
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push ax
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mov ax, 0e0dh
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int 10h
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mov ax, 0e0ah
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int 10h
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pop ax
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ret
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newline endp
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space proc
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push ax
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mov ax, 0e20h
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int 10h
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pop ax
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ret
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space endp
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far_caller proc
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push ax
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push bx
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mov bx, sp
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mov ax, [ss:bx + 8]
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call hexprint16
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mov al, ':'
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mov ah, 0eh
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int 10h
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pop bx
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pop ax
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near_caller proc
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push ax
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push bx
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mov bx, sp
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mov ax, [ss:bx + 6]
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call hexprint16
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call space
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pop bx
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pop ax
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ret
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near_caller endp
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far_caller endp
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logaddr proc
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push ax
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push bx
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mov al, '>'
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mov ah, 0eh
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int 10h
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mov bx, sp
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mov ax, [ss:bx + 4]
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call hexprint16
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call space
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pop bx
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pop ax
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ret
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logaddr endp
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code ends
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end
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