PCjr (Was Re: IBM PC hacking)
chenmel at earthlink.net
Thu Sep 29 19:22:43 CDT 2005
On Thu, 29 Sep 2005 09:33:32 -0700 (PDT)
"Dwight K. Elvey" <dwight.elvey at amd.com> wrote:
> >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
> >250,000 bps. If the data is presented one byte at a time, that is
> >31250 bytes per second. Even with multiple I/O clock cycles per byte
> >get it to the processor and then to memory, a 4.77Mhz processor can
> >up with that easily.
> >Where the lack of DMA would hurt you is on a much faster interface,
> >as a hard disk. Then you wouldn't have any cycles to spare, the the
> >difference would show up.
> 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.
Yep. The floppy disk interface on the PC Junior is painfully slow
because all data has to pass through the CPUs accumulator.
More information about the cctech