looking for help

[woonym]

hi all ! I'm newbie and hope below questions can be answered. thanks

%TITLE "Send printer form feed command -- by Tom Swan"

IDEAL

MODEL Small
STACK 256

;--------- Equates

ASCIIcr         EQU     13      ;Ascii carriage return
ASCIIff         EQU     12      ;Ascii form feed control code

CODESEG

Start:
        mov     ax, @data       ;Initialize DS to address
        mov     ds, ax          ; of data segment

        mov     dl, ASCIIcr     ;Assign cr code to dl
        mov     ah, 05h         ;Dos Function: Printer output
        int     21h             ;Call Dos -- carrige return

        mov     dl, ASCIIff     ;Assign ff code to dl
        mov     ah, 05h         ;Dos Function: Printer output
        int     21h             ;Call Dos -- form feed

Exit:
        mov     ax, 04C00h      ;Dos Function: Exit Program
        int     21h             ;Call Dos. Terminate program

        END     Start           ;End of program / entry point

This is a sample listing of the book "Mastering Turbo Assembler" and my questions are :-


1) whats the meaning of  @data ?

2) why the ASCII code was assigned to dl and not dh ?

3) when dos function 05h was assigned to ah why the value of dx changed to 000D (this was examined in Turbo Debugger) ?


thanks

YM

[MichaelW]

1) @data is a pre-defined symbol for the segment address of the data segment. For a .EXE program the system has no way of knowing what the segment address of the data segment is, so it sets DS (and ES) to the segment address of the Program Segment Prefix, a 256-byte data structure that precedes the program in memory. It's up to the program to load the segment address of the data segment into DS.

2) Interrupt 21h Function 05h expects the ASCII value of the character to print in DL. For information on the DOS interrupts see Ralf Brown's Interrupt list.

An HTML version is here:

http://www.ctyme.com/rbrown.htm

And the download version here:

http://www-2.cs.cmu.edu/~ralf/files.html

3) The decimal value 13 is 0D in hexadecimal.

[woonym]

Pls help me to answer below questions. thanks

%TITLE "Mov demo -- by Tom Swan

IDEAL

MODEL small

STACK 256

DATASEG

exCode          DB      0
speed           DB      99      ;One - byte variable

CODESEG

Start:

        mov     ax, @data        ;Initialize DS to
        mov     ds, ax           ;of data segment

        mov     ax, 1            ;Move immediate data into reg
        mov     bx, 2
        mov     cx, 3
        mov     dx, 4

        mov     ah, [speed]      ;Load value of speed into al
        mov     si, offset speed ;Load address of speed into al

Exit:
        mov     ah, 04Ch         ;Dos Function : Exit Program
        mov     al, [exCode]     ;Return exit code value
        int     21h              ;Call Dos. Terminate program

        END     Start            ;End of program / entry point

1) In this statement,  "mov ah, [speed]" why ah should be used and not al ?

2) what is the relationship of si and ax in this statement " mov si, offset speed" ?

YM

[MichaelW]

1) Either would be legal, and there is nothing in the code to indicate that one should be selected over the other.

2) There is no relationship between AX and SI that I can see, beyond them both being 16-bit registers.

[dedndave]

        mov     ah, [speed]      ;Load value of speed into al
        mov     si, offset speed ;Load address of speed into al

It appears to me that the registers are being set up to read the location at [SI]
into the AL register with a LODSB instruction.

You have to show us the rest of the code - this is only a piece of it.