VESA DrawString.asm

ORG 0h

;DB 0CCh    ;INT 3

PUSH DS         ;Screen will be DS:DI

MOV DS, [wVesaSegment]  ;Load screen segment into DS
            ;DI passed with pointer to start of print area

LGS SI, [FontTablePtr]  ;Load pointer to font table into GS:SI

;String passed on FS:BP
;Length passed on CX
;Color passed on BL


ReadChar:
MOV AH, 00h     ;clear AH for pointer math
MOV AL, BYTE [FS:BP]    ;Read a char from the string
SHL AX, 3       ;Multiply character by 8 (8 bytes per char)

MOV DX, SI      ;Save start position of font table
ADD SI, AX      ;Move table pointer to character we just read

MOV BH, 8       ;8 lines per char

.NextLine:
MOV AL, [ES:SI]     ;Read a row of pixels from the char

.Pixel1:
TEST AL, 1      ;Is there a pixel at bit 1?
JZ .Pixel2      ;No? Jump to pixel 2
MOV [DI+7], BL      ;Draw pixel 1

.Pixel2:
TEST AL, 2      ;Is there a pixel at bit 2?
JZ .Pixel3      ;No? jump to pixel 3

MOV [DI+6], BL      ;Draw pixel 2

.Pixel3:
TEST AL, 4      ;Is there a pixel at bit 3?
JZ .Pixel4      ;No? Jump to pixel 4
MOV [DI+5], BL      ;Draw pixel 3

.Pixel4:
TEST AL, 8      ;Is there a pixel at bit 4?
JZ .Pixel5      ;No? Jump to pixel 5
MOV [DI+4], BL      ;Draw pixel 4

.Pixel5:
TEST AL, 16     ;Is there a pixel at bit 5?
JZ .Pixel6      ;No? Jump to pixel 6
MOV [DI+3], BL      ;Draw pixel 5

.Pixel6:
TEST AL, 32     ;Is there a pixel at bit 6?
JZ .Pixel7      ;No? Jump to pixel 7
MOV [DI+2], BL      ;Draw pixel 6

.Pixel7:
TEST AL, 64     ;Is there a pixel at bit 7?
JZ .Pixel8      ;No? Jump to pixel 8
MOV [DI+1], BL      ;Draw pixel 7

.Pixel8:
TEST AL, 128        ;Is there a pixel at bit 8?
JZ .EndOfRow        ;No? Skip drawing pixel
MOV [DI], BL        ;Draw pixel 8

.EndOfRow:
INC SI          ;Next byte on font table
ADD DI, 640     ;Go to next line on bank

DEC BH          ;Done all 8 lines of char?
JNZ .NextLine       ;No? Start over on next line

MOV SI, DX      ;Restore to start of table
SUB DI, 2560-8      ;Done a char, return to top line and move
            ;8 pixels forward for next character

INC BP          ;Next char in string pointer
DEC CX          ;Are we done the string?
JNZ ReadChar        ;No? Repeat until done


End:
POP DS
RET