26 Oct
2010
26 Oct
'10
5:13 p.m.
Charles Dickman wrote:
The topic of why split octal was used on some early
micro-computers came up
on a list. After doing a search for a TTL LED driver that does hex, the
answer is probably that there were no single chip option for hex displays.
Split octal was just easier.
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.
Note that the Z80 uses a superset of the instruction encodings of the
8080, so the same argument would apply, but Zilog pushed the use of
hexadecimal.
Eric