microcoding a PC into a PDP-11 (was: RE: Classic Computers vs.

From: Sean 'Captain Napalm' Conner <spc_at_conman.org>
Date: Sat Sep 22 19:27:34 2001

It was thus said that the Great Bob Shannon once stated:
>
> A PERQ running POS microcode is simply a PERQ running with one instrucion set, and a
> PERQ running PNX is running a different instuction set. Because both instruction sets
> are running at the same level of hardware abstraction, and not mimicing another
> machine, neither is an emulator!
>
> Emulators are simply mimics of other machines, however they are implemented.

  But the PERQ with the POS microcode is emulating a POS machine! And an
Apple ][ with the POS environment is emulation a POS machine! One is in
hardware, one in software. The only difference is one of speed really.

  The Tandy Color Computer has three one bit ports for serial I/O. One for
carrier detect, one for transmission and one for receiving. To send one
byte using 8n1 at 300 baud, you have to send the start bit for .003 of a
second, then send out each bit to the one bit port and hold the line steady
for .003 of a second, then the stop bit for .003 of a second.

  On the IBM PC (and compatibles) you have access to a UART (an 8250 or 8250
compatible device) and all you have to do is set the chip to send 8n1, set
the bps, and then once that is set, you check the status to see if it can
send, and once it's clear to send, you write the byte to a register.

  Software. Hardware. One is slower, but more flexible (I can do 9e3 at
472 bps if I need to) while the other one is faster but less flexible (I
can't do 9e3 at 472).

  And what you can do in software, you can do in hardware, and vice-versa.

  -spc (It's interpreters all the way down!)
Received on Sat Sep 22 2001 - 19:27:34 BST