29 Sep
2005
29 Sep
'05
9:33 a.m.
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