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From:	pete AT nospam DOT demon DOT co DOT uk
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Subject:	Re: NASM version of John Santic's stderr to stdout TSR
Date:	Tue, 15 Feb 2011 06:17:27 +0000 (UTC)
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In article <hq5193$5pq$1 AT speranza DOT aioe DOT org>
           do_not_have AT havenone DOT cmm "Rod Pemberton" writes:

> The code below is a binary compatible NASM conversion
> of John Santic's stderr to stdout TSR from:
> http://johnsantic.com/comp/tsr.html
> 
> 
> Rod Pemberton
> 
> 
> ; STDERROR.ASM
> 
> BITS 16
> ORG 0x100                                 ; require for .COM file
> SECTION .text
> 
> start:  jmp      short start1
>         nop
> 
> old_vector: dw   00h,00h                  ; save old DOS int 21 vector
> 
> start1:
>         mov      ax, 0
>         mov      es, ax
>         les      bx, [es:21h * 4]         ; read the vector for DOS int 21
>         mov      [old_vector], bx         ; save it so we can chain to it
>         mov      [old_vector + 2], es
>         mov      ax, 0
>         mov      es, ax
>         cli                            ; disable interrupts while we write
> 
>         mov      word [es:21h * 4], isr   ; chain our handler to DOS int 21
>         mov      [es:(21h * 4) + 2], cs
>         sti                               ; enable interrupts
>         mov      ax, 3100h                ; function code to become resident
>         mov      dx, last / 16 + 11h      ; reserve memory paragraphs for us
>         int      21h                   ; return to DOS but remain resident
> 
> ; This interrupt service routine is chained to the main DOS int 21 vector.
> 
> isr:    cmp      ah, 40h                  ; function code = write to file?
>         jne      exit                     ; jump if no, don't do anything
>         cmp      bx, byte 2               ; handle = stderr?
>         jne      exit                     ; jump if no, don't do anything
>         dec      bx                       ; force handle to stdout
> exit:   jmp      far [cs:old_vector]      ; let DOS do its thing
> 
> last    equ      $ - start                ; how much memory we use
> 

I'm not sure if the main aim was binary or functional compatibility, 
but locating the initialisation/setup code at the end will result in a 
smaller memory footprint of the resident code.  I.e.

start:  jmp      short start1
        nop             ;;-------> I guess this is for compat??

old_vector: dw   00h,00h                  ; save old DOS int 21 vector

; This interrupt service routine is chained to the main DOS int 21 vector.

isr:    cmp      ah, 40h                  ; function code = write to file?
        jne      exit                     ; jump if no, don't do anything
        cmp      bx, byte 2               ; handle = stderr?
        jne      exit                     ; jump if no, don't do anything
        dec      bx                       ; force handle to stdout
exit:   jmp      far [cs:old_vector]      ; let DOS do its thing

last    equ      $ - start                ; how much memory we use

        ; ----> we don't need to keep this code resident
start1:
        mov      ax, 0
        mov      es, ax
        les      bx, [es:21h * 4]         ; read the vector for DOS int 21
        mov      [old_vector], bx         ; save it so we can chain to it
        mov      [old_vector + 2], es
        mov      ax, 0
        mov      es, ax
        cli                            ; disable interrupts while we write

        mov      word [es:21h * 4], isr   ; chain our handler to DOS int 21
        mov      [es:(21h * 4) + 2], cs
        sti                               ; enable interrupts
        mov      ax, 3100h                ; function code to become resident
        mov      dx, last / 16 + 11h      ; reserve memory paragraphs for us
        int      21h                   ; return to DOS but remain resident

As a final comment, the ISR itself could be made even smaller :)

; This interrupt service routine is chained to the main DOS int 21 vector.

isr:    cmp      ah, 40h                  ; function code = write to file?
        jne      exit                     ; jump if no, don't do anything
        cmp      bx, byte 2               ; handle = stderr?
        jne      exit                     ; jump if no, don't do anything
        dec      bx                       ; force handle to stdout
exit:   db      0xEA                      ; "jmp far immediate" opcode
old_vector: dw   00h,00h                  ; save old DOS int 21 vector

Pete
-- 
   "We have not inherited the earth from our ancestors,
    we have borrowed it from our descendants."

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