Msg: 1472 *Conference*

02-04-92 09:02:47

From: DREW BLANCHAR

To : MITCH STOCKER

Subj: REPLY TO MSG #1412 (NEW)

Dear Mitch,
  All your questions and your interest are good and healthy.  Don't let anybody
scare you away from asking. To quote somebody or other, "There are no dumb
questions, only people who stay dumb because they don't ask." Or, to quote
Jimmy Stewart, "I don't care so much if you answer my questions, but you
question my answers."
 
David Lein's (possible spelling error) is the one that taught me BASIC and MDL
100 from the batteries up.  But, Robert Benson has a different view since he
started from a different point than I did.  Great book! Too bad it's out of
print. (No, Rick, I won't write my own without something a little wider than
this wonderful example of portability--M DL 100.)
 
  Now, why can't we handle more than 32K at a time?  Even the MDL 200 handles
only 32K at a time, but can swap banks (like exchanging the whole diskette or
tape in the drive).
...... The Central Processing Unit CPU) is the chip that is actually the
computer.  Everything else is peripheral (human interface like screen,
keyboard, disk drive), periferal interface (time and power buffers), power (off
in the corner of the block diagram) and glue (software, stand-alone-programmed
parts, memory).  
  The CPU has address lines (pins and logic) numbering only 0-16.. 2 to the
17th power = 65,536 or 64K (1K = 2 to the 10th = 1024).  32K for ROM and 32K
for RAM is the maximum number of memory address locations that the CPU chip can
handle (we're talking about an 80c85 in these machines).
  
 But, if I've got 64K of ROM & RAM, how can I have anything else working with
the CPU? Good question.  The 80c85 has one more pin called I/bar-M (or maybe
M/bar-I) This is like address 18 but switches the CPU between memory ports
in/out mode (bar-M) and hardware ports in/out mode (I for Input..  these are
the keyboard, the screen, the modem, the UART, and the external buss
connector).
  
  
And that's why we can only play with 32K of RAM at a time, unless you re-write
and re-wire the ROM software and chip to change the partitioning.
 
All better technicians are welcome to correct or ellaborate on my attempted
explanation.
   -dB