Do memory expansions add DMA? (and if so, why?)

Hardware questions and modifications

Do memory expansions add DMA? (and if so, why?)

Postby Trixter » Mon Sep 15, 2008 9:32 am

Like the subject says. I'm curious if memory expansions add DMA capability to the machine; furthermore, if they do, I am wondering what the point is, since disk activity is already handled by the CPU.
You're all insane and trying to steal my magic bag!
Trixter
 
Posts: 508
Joined: Mon Sep 01, 2008 12:00 am
Location: Illinois, USA

Re: Do memory expansions add DMA? (and if so, why?)

Postby Brutman » Mon Sep 15, 2008 12:23 pm

Memory expansions don't generally add DMA. They do include their own DRAM refresh controllers though.

The only DMA addon that I know of is one of the Racore Drive II models. Some of them came with DMA capability which required a different floppy controller and some patches to BIOS to make use of the DMA capability.

DMA on a Jr is generally *not* a desirable feature. So little in the OS uses it, and it breaks compatibility with other things. Then again, it might speed up I/O since the normal processor loop for doing I/O ain't so hot.

(In the 286 era the BIOSes abandoned DMA for disk I/O and went back to processor loops. The jr was just ahead of it's time. ;)
Brutman
Site Admin
 
Posts: 931
Joined: Sat Jun 21, 2008 5:03 pm

Re: Do memory expansions add DMA? (and if so, why?)

Postby Trixter » Mon Sep 15, 2008 2:18 pm

Thanks for the clarification. I'm trying to figure out what my programming options are, just brainstorming a few things for my coding project. The lack of DMA, coupled with people's prior history on using a modem with a jr, has me concerned that if I attempt any diskette activity while something else is going on (like playing a tune), that something will halt dead while the disk activity is going.
You're all insane and trying to steal my magic bag!
Trixter
 
Posts: 508
Joined: Mon Sep 01, 2008 12:00 am
Location: Illinois, USA


Return to PCjr Hardware

Who is online

Users browsing this forum: alanh, Brutman and 1 guest