rob@prism.TMC.COM (11/29/89)
> I have an ATI VGA Wonder card, and I'm confused about what the 16-bit >bus actually does for me. I have set the card to enable 16-bit ROM access, >but is this all the extra bus width is good for? I've read up on the >programming details of normal VGA controllers, and it seems that they are >inherently limited to byte-wide accesses to display memory. There doesn't >seem to be any simple or downward-compatible way to perform 16-bit accesses. >If there isn't any way around this for bit-mapped graphics modes, is there >at least a way to do 16-bit accesses in text modes? By and large, you're right. The 16 bit bus interface doesn't gain you as much as you might hope. Most VGAs are internally still 8 bit devices (the latch registers, through which all video accesses must go in graphics mode, are 8 bits). There are a couple of benefits. 16-bit ROM access gets you faster access to the BIOS code, which in turn leads to faster output from programs that use DOS or the BIOS for accessing the display (DOS itself is probably the primary such program). The 16 bit interface to the PC bus offers greater speed when doing word accesses. This usually happens when a STOSW or MOVSW instruction is used to set a character/attribute pair in text mode. On an 8 bit card, this exacts a penalty because the PC bus must split it into 2 8 bit accesses. The extra overhead isn't normally significant, though; even on a slow machine with an 8 bit video card, the entire screen can usually be updated within a single screen refresh in text mode.