Add interrupts module: INT 3 (CC breakpoint) vs INT 21h (DOS services)
NASM sources, DOSBox/FreeDOS Debug/Unicorn harness, captured output and 31 assertions backing the ankurm.com article "INT 3 vs INT 21h in 8086 Assembly". Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh
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Claude Sonnet 5.5
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; fn_tour.asm -- exercises the important INT 21h functions, one labelled line of output each.
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; Run it under DOS (the harness uses DOSBox) with stdin redirected from IN.TXT.
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bits 16
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org 0x100
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; ---- shrink our memory block: a .COM owns all conventional memory (AH=4Ah) ----
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mov bx, 1000h ; keep 64 KB = 1000h paragraphs
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mov ah, 4Ah ; ES = our PSP segment already
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int 21h
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; ---- AH=30h get DOS version ----
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mov dx, l_ver
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call puts
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mov ah, 30h
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int 21h ; AL = major, AH = minor
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push ax
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xor ah, ah
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call putdec
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mov dl, '.'
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mov ah, 02h
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int 21h
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pop ax
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mov al, ah
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xor ah, ah
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call putdec
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call crlf
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; ---- AH=2Ah / AH=2Ch date and time (values change every run, so we only range-check) ----
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mov dx, l_date
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call puts
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mov ah, 2Ah ; CX = year, DH = month, DL = day
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int 21h
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cmp cx, 1980
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jb .baddate
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cmp dh, 12
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ja .baddate
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mov dx, s_ok
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jmp .dateout
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.baddate:
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mov dx, s_bad
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.dateout:
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call puts
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mov ah, 2Ch ; CH = hour, CL = minute, DH = second
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int 21h
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mov dx, s_ok2
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cmp ch, 23
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jbe .timeok
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mov dx, s_bad
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.timeok:
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call puts
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call crlf
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; ---- AH=02h / AH=06h character output ----
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mov dx, l_chr
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call puts
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mov dl, 'A'
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mov ah, 02h
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int 21h
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mov dl, 'B'
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mov ah, 06h ; direct console output (no ^C check)
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int 21h
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call crlf
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; ---- AH=01h then AH=0Ah stdin: one echoed char, then a buffered line ----
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mov dx, l_in
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call puts
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mov ah, 01h ; AL = char read (and echoed)
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int 21h
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mov [ch1], al
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mov dx, buf
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mov ah, 0Ah ; buf[0] = max, buf[1] = count read, buf[2..] = text
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int 21h
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call crlf
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mov dx, l_got
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call puts
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mov al, [ch1]
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call puthex8 ; the character AH=01h returned
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mov dx, l_cnt
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call puts
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xor ah, ah
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mov al, [buf+1] ; number of characters AH=0Ah stored (CR not counted)
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mov bx, ax
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call putdec
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mov byte [buf+2+bx], '$'
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mov dx, l_txt
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call puts
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mov dx, buf+2
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call puts
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call crlf
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; ---- AH=25h / AH=35h set and get an interrupt vector ----
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mov dx, l_vec
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call puts
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mov ax, 2560h
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mov dx, dummy_isr
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int 21h
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mov ax, 3560h
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int 21h ; ES:BX = vector 60h
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mov dx, s_bad
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cmp bx, dummy_isr
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jne .vecout
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mov ax, es
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mov cx, cs
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cmp ax, cx
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jne .vecout
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mov dx, s_ok
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.vecout:
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call puts
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call crlf
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; ---- AH=48h / AH=49h allocate and free memory; failure returns CF=1, AX=8, BX=largest ----
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mov dx, l_mem
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call puts
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mov bx, 0100h ; 100h paragraphs = 4 KB
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mov ah, 48h
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int 21h
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jc .memfail
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mov es, ax ; ES = segment of the new block
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mov ah, 49h
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int 21h
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mov dx, s_ok
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jmp .memout
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.memfail:
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mov dx, s_bad
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.memout:
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call puts
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call crlf
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mov bx, 0FFFFh ; ask for 1 MB: must fail
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mov ah, 48h
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int 21h
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mov [err], ax ; save the results before printing clobbers them
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mov [big], bx
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mov byte [cfch], '0'
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jnc .memcf
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mov byte [cfch], '1'
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.memcf:
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mov dx, l_memfail
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call puts
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mov dl, [cfch]
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mov ah, 02h
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int 21h
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mov dx, l_ax
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call puts
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mov ax, [err] ; 0008h = insufficient memory
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call puthex16
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mov dx, l_big
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call puts
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mov ax, [big] ; BX = largest block available, in paragraphs
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call puthex16
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call crlf
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; ---- file I/O: AH=3Ch create, 40h write, 3Eh close, 3Dh open, 3Fh read, 42h lseek ----
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mov dx, l_file
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call puts
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mov dx, fname
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xor cx, cx ; normal attributes
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mov ah, 3Ch
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int 21h
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jc .fileerr
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mov bx, ax ; BX = handle
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mov dx, payload
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mov cx, payload_len
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mov ah, 40h
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int 21h
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mov ah, 3Eh
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int 21h
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mov dx, fname
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mov ax, 3D00h ; open read-only
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int 21h
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jc .fileerr
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mov bx, ax
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mov dx, rdbuf
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mov cx, 64
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mov ah, 3Fh
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int 21h ; AX = bytes actually read
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call putdec ; bytes actually read
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mov dx, l_read
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call puts
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mov ax, 4202h ; LSEEK from end, offset 0
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xor cx, cx
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xor dx, dx
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int 21h ; DX:AX = file size
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mov [fsize], ax
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mov dx, l_seek
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call puts
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mov ax, [fsize]
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call putdec
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mov ah, 3Eh
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int 21h
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call crlf
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jmp .fileok
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.fileerr:
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mov dx, s_bad
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call puts
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call crlf
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.fileok:
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; ---- AH=56h rename, AH=4Eh findfirst (DTA defaults to PSP:0080h), AH=41h delete ----
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mov dx, l_ff
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call puts
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mov dx, fname
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mov di, fname2
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push ds
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pop es
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mov ah, 56h
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int 21h
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mov dx, fname2
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xor cx, cx
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mov ah, 4Eh
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int 21h
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jc .ffbad
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mov dx, 80h+1Eh ; DTA+1Eh = ASCIZ name found
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mov si, dx ; copy it to a '$'-terminated buffer so AH=09h can print it
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mov di, ffname
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.cp:
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lodsb
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test al, al
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jz .cpend
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stosb
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jmp .cp
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.cpend:
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mov al, '$'
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stosb
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mov dx, ffname
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call puts
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mov dx, l_size
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call puts
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mov ax, [80h+1Ah] ; DTA+1Ah = file size (low word)
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call putdec
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jmp .ffend
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.ffbad:
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mov dx, s_bad
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call puts
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.ffend:
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call crlf
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mov dx, l_del
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call puts
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mov dx, fname2
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mov ah, 41h ; delete
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int 21h
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mov dx, fname2
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mov ax, 3D00h ; open it again: must fail
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int 21h
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mov [err], ax ; save AX and CF before printing clobbers them
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mov byte [cfch], '0'
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jnc .cfout
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mov byte [cfch], '1'
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.cfout:
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mov dl, [cfch]
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mov ah, 02h
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int 21h
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mov dx, l_ax
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call puts
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mov ax, [err]
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call puthex16
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call crlf
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; ---- AH=39h mkdir, AH=3Ah rmdir, AH=19h current drive, AH=47h current directory ----
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mov dx, l_dir
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call puts
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mov dx, dname
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mov ah, 39h
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int 21h
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mov dx, dname
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mov ah, 3Ah
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int 21h
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mov dx, s_ok
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jnc .dirout
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mov dx, s_bad
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.dirout:
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call puts
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mov dx, l_drv
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call puts
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mov ah, 19h ; AL = 0 for A:, 1 for B:, 2 for C:
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int 21h
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xor ah, ah
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call putdec
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call crlf
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; ---- AH=4Bh EXEC a child, AH=4Dh read its return code ----
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mov dx, l_exec
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call puts
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mov ax, cs
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mov [pb+4], ax ; command-tail segment
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mov [pb+8], ax ; FCB1 segment
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mov [pb+12], ax ; FCB2 segment
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mov dx, child
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mov bx, pb
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mov ax, 4B00h ; load and execute
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int 21h
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jc .execbad
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mov ah, 4Dh ; AL = child's return code, AH = termination type
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int 21h
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push ax
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mov dx, l_rc
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call puts
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pop ax
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push ax
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xor ah, ah
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call putdec
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mov dx, l_tt
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call puts
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pop ax
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mov al, ah
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xor ah, ah
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call putdec
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call crlf
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jmp .done
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.execbad:
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mov dx, s_bad
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call puts
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call crlf
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.done:
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mov ax, 4C00h ; AH=4Ch: exit, return code 0
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int 21h
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dummy_isr:
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iret
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l_ver db 'AH=30h DOS version : $'
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l_date db 'AH=2Ah/2Ch date+time ranges : $'
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l_chr db 'AH=02h + AH=06h : $'
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l_in db 'AH=01h + AH=0Ah (stdin) : echo -> $'
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l_got db ' AH=01h got 0x$'
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l_txt db ', text = $'
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l_cnt db ' ; AH=0Ah count = $'
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l_vec db 'AH=25h/35h vector 60h : $'
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l_mem db 'AH=48h/49h alloc+free 4 KB : $'
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l_memfail db 'AH=48h ask for 1 MB (fails) : CF=$'
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l_big db ' ; largest block (paragraphs) = $'
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l_file db 'AH=3Ch/40h/3Dh/3Fh/42h : $'
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l_read db ' bytes read ; $'
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l_ff db 'AH=56h + AH=4Eh findfirst : $'
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l_size db ' size = $'
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l_del db 'AH=41h delete, reopen fails : CF=$'
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l_seek db 'LSEEK(end) says size = $'
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l_ax db ' AX=$'
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l_dir db 'AH=39h/3Ah mkdir+rmdir : $'
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l_drv db ' ; AH=19h drive = $'
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l_exec db 'AH=4Bh exec child : $'
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l_rc db 'AH=4Dh return code = $'
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l_tt db ' type = $'
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s_ok db 'ok$'
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s_ok2 db ' ok$'
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s_bad db 'FAILED$'
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fname db 'T.TXT', 0
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fname2 db 'U.TXT', 0
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dname db 'SUBD', 0
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child db 'HELLO21.COM', 0
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payload db 'DOS file I/O'
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payload_len equ $ - payload
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ch1 db 0
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err dw 0
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fsize dw 0
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big dw 0
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cfch db 0
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buf db 20, 0
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times 22 db 0
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ffname times 16 db 0
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rdbuf times 64 db 0
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; EXEC parameter block: env seg (0 = inherit), cmd tail ptr, FCB1 ptr, FCB2 ptr
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pb dw 0, tail, 0, fcb1, 0, fcb2, 0
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tail db 0, 0Dh
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fcb1 times 16 db 0
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fcb2 times 16 db 0
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%include "common.inc"
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