27 Oct
2010
27 Oct
'10
7:11 a.m.
On 10/26/10 8:40 PM, Chuck Guzis wrote:
The IMSAI 8080 also has both octal and hexadecimal groupings above
the first row of LEDs, printed on the sheet that sits between the two
parts of the plexiglas bezel.
-Dave
--
Dave McGuire
Port Charlotte, FL
The 8080
instruction encoding is based on 3 bit fields for register
selection, so octal is natural. Split octal for 16-bit addresses
resulted from the way 16-bit addresses were stored in memory as two
consecutive bytes. If a JMP instruction was stored in memory in octal
as 303 222 111, it was more natural to consider the address to be 111
222 in split octal rather than 044622 in 16-bit octal.
When I wrote a cross-assembler for the 8008, I used octal. When I
went to the 8080, I used hex. The only difference was the projects I
worked with at the day job--one mainframe used octal; the other used
hex. Both from the same manufacturer.
I liked the IMSAI scheme of color-coding the front panel toggles, so
you could group them by 3 or 4 as your preference demanded.