8-bit ISA Writes on a Pentium

Hi, after reading about the big read penalty for byte transfers from 8-bit ISA slots with Pentium processors, it fetching a full 8 bytes on each access, I'm left wondering how the Pentium handles single-byte writes to the ISA bus?

And also, how does the motherboard know to route a write to (say) D000h to an ISA expansion card so configured anyway?

pearce_jj said:

And also, how does the motherboard know to route a write to (say) D000h to an ISA expansion card so configured anyway?

Click to expand...

Look up the term Subtractive Decode.

A quick search turned up the article, for example:

http://www.tech-pro.net/intro_pci.html

Subtractive decode: Art imitating life: "Are you going to eat that?" or, "If by the time I count to three, nobody claims it, it's mine."



Which explains why PCI-to-ISA bridges are such a pain.

Many thanks!