PCjr (Was Re: IBM PC hacking)

Scott Stevens 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
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
> 
Yep.  The floppy disk interface on the PC Junior is painfully slow
because all data has to pass through the CPUs accumulator.

> 



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