Programmer 7500/MAX_PROGRAMMERS.iso/INFO/PROGJRN/PJ_5_3.ARC/PJ3.ASM

Assembly Source File | 1987-10-12 | 5.0 KB | 222 lines

; Program to illustrate operation of data rotate and bit mask
;  features of Graphics Data Controller.  Draws 8x8 character at
;  specified location, using EGA's 8x8 ROM font.  Designed
;  for use with modes 0Dh, 0Eh, 0Fh, and 10h.
; Assembled with MASM 4.0, linked with LINK 3.51.
; Copyright by Michael Abrash, 2/8/87.
;
stack    segment    para stack 'STACK'
    db    512 dup(?)
stack    ends
;
EGA_VIDEO_SEGMENT    equ    0a000h    ;EGA display memory segment
SCREEN_WIDTH_IN_BYTES    equ    044ah    ;offset of BIOS variable
FONT_CHARACTER_SIZE    equ    8    ;# bytes in each font char
;
; EGA register equates.
;
GDC_INDEX    equ    3ceh    ;GDC index register
GDC_ROTATE    equ    3    ;GDC data rotate/logical function
                ; register index
GDC_BIT_MASK    equ    8    ;GDC bit mask register index
;
dseg    segment    para common 'DATA'
TEST_TEXT_ROW    equ    69    ;row to display test text at
TEST_TEXT_COL    equ    17    ;column to display test text at
TEST_TEXT_WIDTH    equ    8    ;width of a character in pixels
TestString    label    byte
    db    'Hello, world!',0    ;test string to print.
FontPointer    dd    ?        ;font offset
dseg    ends
;
; Macro to set indexed register INDEX of GDC chip to SETTING.
;
SETGDC    macro    INDEX, SETTING
    mov    dx,GDC_INDEX
    mov    ax,(SETTING SHL 8) OR INDEX
    out    dx,ax
    endm
;
cseg    segment    para public 'CODE'
    assume    cs:cseg, ds:dseg
start    proc    near
    mov    ax,dseg
    mov    ds,ax
;
; Select 640x350 graphics mode.
;
    mov    ax,010h
    int    10h
;
; Set driver to use the 8x8 font.
;
    mov    ah,11h    ;EGA BIOS character generator function,
    mov    al,30h    ; return info subfunction
    mov    bh,3    ;get 8x8 font pointer
    int    10h
    call    SelectFont
;
; Print the test string.
;
    mov    si,offset TestString
    mov    bx,TEST_TEXT_ROW
    mov    cx,TEST_TEXT_COL
StringOutLoop:
    lodsb
    and    al,al
    jz    StringOutDone
    call    DrawChar
    add    cx,TEST_TEXT_WIDTH
    jmp    StringOutLoop
StringOutDone:
;
; Reset the data rotate and bit mask registers.
;
    SETGDC    GDC_ROTATE, 0
    SETGDC    GDC_BIT_MASK, 0ffh
;
; Exit to DOS.
;
    mov    ah,4ch
    int    21h
Start    endp
;
; Subroutine to draw a text character in a linear graphics mode
;  (0Dh, 0Eh, 0Fh, 010h).
; Font used should be pointed to by FontPointer.
;
; Input:
;  AL = character to draw
;  BX = row to draw text character at
;  CX = column to draw text character at
;
;  Forces ALU function to "move".
;
DrawChar    proc    near
    push    ax
    push    bx
    push    cx
    push    dx
    push    si
    push    di
    push    bp
    push    ds
;
; Set DS:SI to point to font and ES to point to display memory.
;
    lds    si,[FontPointer]    ;point to font
    mov    dx,EGA_VIDEO_SEGMENT
    mov    es,dx            ;point to display memory
;
; Calculate screen address of byte character starts in.
;
    push    ds    ;point to BIOS data segment
    sub    dx,dx
    mov    ds,dx
    xchg    ax,bx
    mov    di,ds:[SCREEN_WIDTH_IN_BYTES]    ;retrieve BIOS
                        ; screen width
    pop    ds
    mul    di    ;calculate offset of start of row
    push    di    ;set aside screen width
    mov    di,cx    ;set aside the column
    and    cl,0111b ;keep only the column in-byte address
    shr    di,1
    shr    di,1
    shr    di,1    ;divide column by 8 to make a byte address
    add    di,ax    ;and point to byte
;
; Calculate font address of character.
;
    sub    bh,bh
    shl    bx,1    ;assumes 8 bytes per character; use
    shl    bx,1    ; a multiply otherwise
    shl    bx,1    ;offset in font of character
    add    si,bx    ;offset in font segment of character
;
; Set up the GDC rotation.
;
    mov    dx,GDC_INDEX
    mov    al,GDC_ROTATE
    mov    ah,cl
    out    dx,ax
;
; Set up BH as bit mask for left half,
; BL as rotation for right half.
;
    mov    bx,0ffffh
    shr    bh,cl
    neg    cl
    add    cl,8
    shl    bl,cl
;
; Draw the character, left half first, then right half in the
; succeeding byte, using the data rotation to position the character
; across the byte boundary and then using the bit mask to get the
; proper portion of the character into each byte.
; Does not check for case where character is byte-aligned and
; no rotation and only one write is required.
;
    mov    bp,FONT_CHARACTER_SIZE
    mov    dx,GDC_INDEX
    pop    cx    ;get back screen width
    dec    cx
    dec    cx    ; -2 because do two bytes for each char
CharacterLoop:
;
; Set the bit mask for the left half of the character.
;
    mov    al,GDC_BIT_MASK
    mov    ah,bh
    out    dx,ax
;
; Get the next character byte & write it to display memory.
; (Left half of character.)
;
    mov    al,[si]        ;get character byte
    mov    ah,es:[di]    ;load latches
    stosb            ;write character byte
;
; Set the bit mask for the right half of the character.
;
    mov    al,GDC_BIT_MASK
    mov    ah,bl
    out    dx,ax
;
; Get the character byte again & write it to display memory.
; (Right half of character.)
;
    lodsb            ;get character byte
    mov    ah,es:[di]    ;load latches
    stosb            ;write character byte
;
; Point to next line of character in display memory.
;
    add    di,cx
;
    dec    bp
    jnz    CharacterLoop
;
    pop    ds
    pop    bp
    pop    di
    pop    si
    pop    dx
    pop    cx
    pop    bx
    pop    ax
    ret
DrawChar    endp
;
; Set the pointer to the font to draw from to ES:BP.
;
SelectFont    proc    near
    mov    word ptr [FontPointer],bp    ;save pointer
    mov    word ptr [FontPointer+2],es
    ret
SelectFont    endp
;
cseg    ends
    end    start