PCjr (Was Re: IBM PC hacking)

[Chuck Guzis]

On 9/29/2005 at 9:33 AM Dwight K. Elvey wrote:

> I don't think the early PC's used DMA for the hard disk.
>The HD controllers usually had sector or track buffers.
>The program would wait until the buffer was full and
>then just move it by software to memory. The floppies
>needed DMA because the controllers didn't buffer more
>than one byte. The processor would have had to dedicate
>it self to the one task without interrupts.

PC's went away from DMA in the AT, but the XT used DMA channel 3.  The XT implemented only a two-sector buffer, so DMA for multiple sector reads made sense (on the controller board itself, double-buffering was used).

The XT controllers were (the 10 MB controller was different from the 20MB one)  a peculiar beast with a bunch of proprietary LSI.  When the AT version was implemented, the Western Digital type (WD1000) of controller design was used, with (faster than DMA) programmed I/O transfers.

Cheers,
Chuck