;COLORINIT EQU       03H       ;Value to initialize color screen (80x25)
COLORINIT EQU      01H       ;Value to initialize color screen (40X25)
;
;         Now, describe our own segment
;
SETSCRN   SEGMENT             ;Set operating segment for CODE and DATA
;
          ASSUME CS:SETSCRN,DS:SETSCRN,ES:SETSCRN,SS:SETSCRN    ;All segments
;
          ORG       100H      ;Begin assembly at standard .COM offset
;
MAIN      PROC      NEAR      ;COM files use NEAR linkage
          JMP       BEGIN     ;And, it is helpful to put the data first, but
;                             ;then you must branch around it.
;
;         Data used in SETSCRN
;
CHANGELOC   DD      EQUIP     ;Location of the EQUIP, recorded as far pointer
MONOPROMPT  DB      'Please press the plus ( + ) key.$'    ;User sees on mono
COLORPROMPT DB      'Please press the minus ( - ) key.$'   ;User sees on color


Several things are illustrated on this page.  First, in addition to titles,
the assembler supports subtitles:  hence the SUBTTL pseudo-op.  Second, the
PAGE pseudo-op can be used to go to a new page in the listing.  You see an
example here of the DSECT-style segment in the "SEGMENT AT 40H".  Here, our
our interest is in correctly describing the location of some data in the
BIOS work area which really is located at segment 40H.

You will also see illustrated the EQU instruction, which just gives a sym-
bolic name to a number.  I don't make a fetish of giving a name to every
single number in a program.  I do feel strongly, though, that interrupts
and function codes, where the number is arbitrary and the function being
performed is the thing of interest, should always be given symbolic names.

One last new element in this section is the define doubleword (DD) instruc-
tion.  A doubleword constant can refer, as in this case, to a location in
another segment.  The assembler will be happy to use information at its
disposal to properly assemble it.  In this case, the assembler knows that
EQUIP is offset 10 in the segment BIOSDATA which is at 40H.

          SUBTTL -- Perform function
          PAGE
BEGIN:    CALL      MONOON                  ;Turn on mono display
          MOV       DX,OFFSET MONOPROMPT          ;GET MONO PROMPT
          MOV       AH,PRTMSG                     ;ISSUE
          INT       DOS                           ;IT
          CALL      COLORON                 ;Turn on color display
          MOV       DX,OFFSET COLORPROMPT         ;GET COLOR PROMPT
          MOV       AH,PRTMSG                     ;ISSUE
          INT       DOS                           ;IT
          MOV       AH,GETKEY               ;Obtain user response
          INT       KBD
          CMP       AL,'+'                  ;Does he want MONO?
          JNZ       NOMONO


IBM PC Assembly Language Tutorial                                        26



          CALL      MONOON                  ;yes.  give it to him
NOMONO:   RET
MAIN      ENDP


The main code section makes use of subroutines to keep the basic flow sim-
ple.  About all that's new to you in this section is the use of the BIOS
interrupt KBD to read a character from the keyboard.

Now for the subroutines, MONOON and COLORON:

          SUBTTL -- Routines to turn monitors on
          PAGE
MONOON    PROC      NEAR                ;Turn mono on
          LES       DI,CHANGELOC        ;Get location to change
          ASSUME    ES:BIOSDATA         ;TELL ASSEMBLER ABOUT CHANGE TO ES
          OR        EQUIP,MONO
          MOV       AX,MONOINIT         ;Get screen initialization value
          INT       SCREEN              ;Initialize screen
          RET
MONOON    ENDP
COLORON   PROC      NEAR                ;Turn color on
          LES       DI,CHANGELOC        ;Get location to change
          ASSUME    ES:BIOSDATA         ;TELL ASSEMBLER ABOUT CHANGE TO ES
          AND       EQUIP,COLOR
          MOV       AX,COLORINIT        ;Get screen initialization value
          INT       SCREEN              ;Initialize screen
          RET
COLORON   ENDP
SETSCRN   ENDS                          ;End of segment
          END       MAIN                ;End of assembly; execution at MAIN


The instructions LES and LDS are useful ones for dealing with doubleword
addresses.  The offset is loaded into the operand register and the segment
into ES (for LES) or DS (for LDS).  By telling the assembler, with an
ASSUME, that ES now addresses the BIOSDATA segment, it is able to correctly
assemble the OR and AND instructions which refer to the EQUIP byte.  An ES
segment prefix is added.

To understand the action here, you simply need to know that flags in that
particular byte control how the BIOS screen service initializes the adapt-
ers.  BIOS will only work with one adapter at a time; by setting the equip-
ment flags to show one or the other as installed and calling BIOS screen
initialization, we achieve the desired effect.

The rest is up to you.
