PCjr (Was Re: IBM PC hacking)

[Dwight K. Elvey]

>From: "Michael B. Brutman" <mbbrutman at brutman.com>
>
>
>I don't think that the lack of a DMA controller on the PCjr really 
>slowed it down any.  The old double density drives have a data rate of 
>250,000 bps.  If the data is presented one byte at a time, that is just 
>31250 bytes per second.  Even with multiple I/O clock cycles per byte to 
>get it to the processor and then to memory, a 4.77Mhz processor can keep 
>up with that easily.
>
>Where the lack of DMA would hurt you is on a much faster interface, such 
>as a hard disk.  Then you wouldn't have any cycles to spare, the the 
>difference would show up.

Hi
 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.
Dwight