jlt@meccts.UUCP (James L. Thompson) (11/18/85)
Does anyone know how to determine if the Color Graphics Adapter is performing a vertical retrace? I have written a routine to display text by directly addressing the screen memory. When I use this routine on the monochrome card I don't get any snow. When I use this on the color card, however, it looks like hell. The Tech. Manual states '...the CPU should access the display buffer during the vertical retrace time(if not, then the display will be affected with random patterns...)'. It also refers to vertical retrace as Interrupt Level 2 (I assume that means the IRQ2 interrupt line). After looking at the schematics I could find no connection to any IRQ line on the bus. This leads me to believe that the only retrace that can be access by software is the horizontal retrace (IBM calls it Display Enable or Horizontal Drive) via 6845 port X'3DA', bit 0. The horizontal retrace does not provide sufficient time to display a whole line and therefore must be called for each character displayed. This technique is just too slow.
ching@amd.UUCP (Mike Ching) (11/21/85)
In article <> jlt@meccts.UUCP (James L. Thompson) writes: > > Does anyone know how to determine if the Color Graphics Adapter is >performing a vertical retrace? The BIOS tests bit 3 of the 6845 status register (3DAH). When this bit is active, the raster is in vertical retrace and the video can be turned off by clearing bit 3 of the 6845 mode select register (3D8H). Reenable video when the update is complete.
spud@olivee.UUCP (John Purser) (11/24/85)
Here's what I use to find vertical retrace. The proc exits on vertical retrace. STATUS_PORT EQU 03DAh V_RTRC EQU 8 vrt_rtrc proc near push ax push dx mov dx,STATUS_PORT l01: in al,dx ; stays in this loop if you enter during test al,V_RTRC ; vertical retrace jnz l01 l02: in al,dx ; stays in this loop until you hit test al,V_RTRC ; vertical retrace then exits jz l02 pop dx pop ax ret vrt_rtrc endp This works on an ATT6300 and should work on an IBM. The top loop is there so that I won't exit until I have a full vertical retrace to use. John Purser Olivetti ATC