Last Updated or created 2023-10-05
Not interesting for most of you, but here is the minimal code to display a line using toggling the background color at a specific retrace.
3C8h (R/W): DAC Address Write Mode bit 0-7 The color data register (0..255) to be written to 3C9h. Note: After writing the 3 bytes at 3C9h this register will increment, pointing to the next data register. 3C9h (R/W): DAC Data Register bit 0-8? Color value Note: Each read or write of this register will cycle through first the registers for Red, Blue and Green, then increment the appropriate address register, thus the entire palette can be loaded by writing 0 to the DAC Address Write Mode register 3C8h and then writing all 768 bytes of the palette to this register. 3DAh Input Status #1 Register (Read at 3BAh (mono) or 3DAh (color)) 7 6 5 4 3 2 1 0 VRetrace DD VRetrace -- Vertical Retrace "When set to 1, this bit indicates that the display is in a vertical retrace interval.This bit can be programmed, through the Vertical Retrace End register, to generate an interrupt at the start of the vertical retrace." DD -- Display Disabled "When set to 1, this bit indicates a horizontal or vertical retrace interval. This bit is the real-time status of the inverted 'display enable' signal. Programs have used this status bit to restrict screen updates to the inactive display intervals in order to reduce screen flicker. The video subsystem is designed to eliminate this software requirement; screen updates may be made at any time without screen degradation."
use16 org 0x100 INPUT_STATUS = 0x03DA start: ; set mode 320x200 256 colors palette mov ah,0x0 mov al,13h int 10h ; press key to exit waitforkeyloop: call effect ; Calling the effect MOV AH,1 INT 16h JZ waitforkeyloop XOR AH,AH INT 16h Exit: MOV AX,3 ; default text mode 3 INT 10h MOV AX,4C00h ; exit to dos (terminate process) INT 21h effect: cli ; stop interrupts call waitvretrace ; wait for vertical retrace mov al, 0 ; set color index 0 to black (needs to be converted to a function mov dx, 3c8h out dx, al inc dx ; now 3c9h mov al, 0h out dx, al ; set R = 0 mov al, 0h out dx, al ; set G = 0 mov al, 0h out dx, al ; set B = 0 mov al,30h ; al = scanline, call wait for scanline call waithretrace mov al, 0 ; set color index 0 to white mov dx, 3c8h out dx, al inc dx mov al, 255 out dx, al mov al, 255 out dx, al mov al, 255 out dx, al ; wait 1 scanlines (height of bar) mov al,1h call waithretrace ; draw black again mov al, 0 ; set color index 0's rgb value mov dx, 3c8h out dx, al inc dx ; now 3c9h mov al, 0 out dx, al out dx, al out dx, al sti ; start interrupts again ret ; this waits for vertical retrace waitvretrace: mov dx,INPUT_STATUS waitv1: in al,dx test al,8 jnz waitv1 waitv2: in al,dx test al,8 jz waitv2 ret ; routine that waits for horizontal retrace ; al sets number of retraces waithretrace: mov cl,al mov dx,INPUT_STATUS waith1: in al,dx test al,1 jnz waith1 waith2: in al,dx test al,1 jz waith2 dec cl cmp cl,0 jnz waith1 ret
My bash file to copy com file to floppy image to use in PCem.
PCem right button disk change drive A:
fasm one-line.asm # disk.img is een msdos boot floppy image sudo mount -o loop disk.img mountpoint sudo cp *com mountpoint/ sudo cp *bmp mountpoint/ sudo umount mountpoint