21 Nov
2009
21 Nov
'09
6:57 p.m.
That link is quite interesting, especially the
pictures. From the way
the front panel is laid out, I would assume values are stated in octal
rather than hex. The bits on the panel seem to have a 2-3-3 pattern so
hex FF would be thought of as octal 377. This appears to be true of the
2 address bytes as well. I think this means that for example address
hex FFFF would be represented as 377,377. That would take some real
That ecame failry common on early 8-bit microprocessor developement
systems. If you had to load the address as 2 separae bytes it was
actually a lot easier to handle each of them separately as an octal
number from 0-377.
I mentioned the HP9830 in another message. The microcode on that machine
is 256 words long, oviously with an 8 bit address. But owing the the
hardware design of the processor, you consider it as 2 separate nybbles
(belice-it-or-not, one of the 4-bit fields in the microode word that is
used to determine the next address [1] also selects the branch
condition). Anyway, the patents for this series of machines, which
contain a tremendous amount of information about them, give those address
as _4 digit_ octal numbers. Each nybble is represented by a number in the
range 0-17 (octal). So, for example hex E3 becomes 1603. It's not hard to
get used to...
[1] Being an strange HP machine, the fields that are used to determine
the next address don't simply contain the next address. Rather, they tell
the microocde program counter which bits to change to get the new address
(effectively, the next address fields are XORed with the current address
to get the next one. Yes I do work on these machines at this sort of level.
-tony