Category : Assembly Language Source Code
Archive   : NORTASM.ZIP
Filename : VIDEO_18.ASM

.MODEL SMALL
.CODE

PUBLIC WRITE_STRING
;-----------------------------------------------------------------------;
; This procedure writes a string of characters to the screen. The ;
; string must end with DB 0 ;
; ;
; On entry: DS:DX Address of the string ;
; ;
; Uses: WRITE_CHAR ;
;-----------------------------------------------------------------------;
WRITE_STRING PROC
PUSH AX
PUSH DX
PUSH SI
PUSHF ;Save direction flag
CLD ;Set direction for increment (forward)
MOV SI,DX ;Place address into SI for LODSB
STRING_LOOP:
LODSB ;Get a character into the AL register
OR AL,AL ;Have we found the 0 yet?
JZ END_OF_STRING ;Yes, we are done with the string
MOV DL,AL ;No, write character
CALL WRITE_CHAR
JMP STRING_LOOP
END_OF_STRING:
POPF ;Restore direction flag
POP SI
POP DX
POP AX
RET
WRITE_STRING ENDP

PUBLIC WRITE_HEX
;-----------------------------------------------------------------------;
; This procedure converts the byte in the DL register to hex and writes ;
; the two hex digits at the current cursor position. ;
; ;
; On entry: DL Byte to convert to hex. ;
; ;
; Uses: WRITE_HEX_DIGIT ;
;-----------------------------------------------------------------------;
WRITE_HEX PROC ;Entry point
PUSH CX ;Save registers used in this procedure
PUSH DX
MOV DH,DL ;Make a copy of byte
MOV CX,4 ;Get the upper nibble in DL
SHR DL,CL
CALL WRITE_HEX_DIGIT ;Display first hex digit
MOV DL,DH ;Get lower nibble into DL
AND DL,0Fh ;Remove the upper nibble
CALL WRITE_HEX_DIGIT ;Display second hex digit
POP DX
POP CX
RET
WRITE_HEX ENDP

PUBLIC WRITE_HEX_DIGIT
;-----------------------------------------------------------------------;
; This procedure converts the lower 4 bits of DL to a hex digit and ;
; writes it to the screen. ;
; ;
; On entry: DL Lower 4 bits contain number to be printed ;
; in hex. ;
; ;
; Uses: WRITE_CHAR ;
;-----------------------------------------------------------------------;
WRITE_HEX_DIGIT PROC
PUSH DX ;Save registers used
CMP DL,10 ;Is this nibble <10?
JAE HEX_LETTER ;No, convert to a letter
ADD DL,"0" ;Yes, convert to a digit
JMP Short WRITE_DIGIT ;Now write this character
HEX_LETTER:
ADD DL,"A"-10 ;Convert to hex letter
WRITE_DIGIT:
CALL WRITE_CHAR ;Display the letter on the screen
POP DX ;Restore old value of DX
RET
WRITE_HEX_DIGIT ENDP

PUBLIC WRITE_CHAR
EXTRN CURSOR_RIGHT:PROC
;-----------------------------------------------------------------------;
; This procedure outputs a character to the screen using the ROM BIOS ;
; routines, so that characters such as the backspace are treated as ;
; any other character and are displayed. ;
; ;
; This procedure must do a bit of work to update the cursor position. ;
; ;
; On entry: DL Byte to print on screen. ;
; ;
; Uses: CURSOR_RIGHT ;
;-----------------------------------------------------------------------;
WRITE_CHAR PROC
PUSH AX
PUSH BX
PUSH CX
PUSH DX
MOV AH,9 ;Call for output of character/attribute
MOV BH,0 ;Set to display page 0
MOV CX,1 ;Write only one character
MOV AL,DL ;Character to write
MOV BL,7 ;Normal attribute
INT 10h ;Write character and attribute
CALL CURSOR_RIGHT ;Now move to next cursor position
POP DX
POP CX
POP BX
POP AX
RET
WRITE_CHAR ENDP

PUBLIC WRITE_DECIMAL
;-----------------------------------------------------------------------;
; This procedure writes a 16-bit, unsigned number in decimal notation. ;
; ;
; On entry: DX N : 16-bit, unsigned number. ;
; ;
; Uses: WRITE_HEX_DIGIT ;
;-----------------------------------------------------------------------;
WRITE_DECIMAL PROC
PUSH AX ;Save registers used here
PUSH CX
PUSH DX
PUSH SI
MOV AX,DX
MOV SI,10 ;Will divide by 10 using SI
XOR CX,CX ;Count of digits placed on stack
NON_ZERO:
XOR DX,DX ;Set upper word of N to 0
DIV SI ;Calculate N/10 and (N mod 10)
PUSH DX ;Push one digit onto the stack
INC CX ;One more digit added
OR AX,AX ;N = 0 yet?
JNE NON_ZERO ;Nope, continue
WRITE_DIGIT_LOOP:
POP DX ;Get the digits in reverse order
CALL WRITE_HEX_DIGIT
LOOP WRITE_DIGIT_LOOP
END_DECIMAL:
POP SI
POP DX
POP CX
POP AX
RET
WRITE_DECIMAL ENDP

PUBLIC WRITE_CHAR_N_TIMES
;-----------------------------------------------------------------------;
; This procedure writes more than one copy of a character ;
; ;
; On entry: DL Character code ;
; CX Number of times to write the character ;
; ;
; Uses: WRITE_CHAR ;
;-----------------------------------------------------------------------;
WRITE_CHAR_N_TIMES PROC
PUSH CX
N_TIMES:
CALL WRITE_CHAR
LOOP N_TIMES
POP CX
RET
WRITE_CHAR_N_TIMES ENDP

PUBLIC WRITE_PATTERN
;-----------------------------------------------------------------------;
; This procedure writes a line to the screen, based on data in the ;
; form ;
; ;
; DB {character, number of times to write character}, 0 ;
; Where {x} means that x can be repeated any number of times ;
; ;
; On entry: DS:DX Address of the pattern to draw ;
; ;
; Uses: WRITE_CHAR_N_TIMES ;
;-----------------------------------------------------------------------;
WRITE_PATTERN PROC
PUSH AX
PUSH CX
PUSH DX
PUSH SI
PUSHF ;Save the direction flag
CLD ;Set direction flag for increment
MOV SI,DX ;Move offset into SI register for LODSB
PATTERN_LOOP:
LODSB ;Get character data into AL
OR AL,AL ;Is it the end of data (0h)?
JZ END_PATTERN ;Yes, return
MOV DL,AL ;No, set up to write character N times
LODSB ;Get the repeat count into AL
MOV CL,AL ;And put in CX for WRITE_CHAR_N_TIMES
XOR CH,CH ;Zero upper byte of CX
CALL WRITE_CHAR_N_TIMES
JMP PATTERN_LOOP
END_PATTERN:
POPF ;Restore direction flag
POP SI
POP DX
POP CX
POP AX
RET
WRITE_PATTERN ENDP

END