21 Nov
2016
21 Nov
'16
12:29 a.m.
On 11/20/2016 05:30 PM, Charles Dickman wrote:
On Fri, Nov 18, 2016 at 6:15 PM, Jon Elson <elson
at pico-systems.com> wrote:
OK, in this manual :
http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/fe/2020/Z26-5908_3…
They describe the functions available in microcode. They
are REALLY limited!
See Page 1-23, Description, item 4. They have data transfer
and increment and decrement.
Period! I don't see ANYTHING that relates to ALU
functions! Maybe I missed it, it seems really hard to believe.
and then page 1-27 describing what the increment and
decrement commands do. Notably, they note that the
registers contain values from 0-15, ie. a 4-bit field.
Maybe I missed the whole point of what they are describing
here, but this seems to be somewhat consistent with what you
can pack into a 16-bit microinstruction, namely, not a lot.
Take a look in this manual and see if you get a different
impression.
Jon
OH, yeah! Besides the limited instruction set,
short registers only half
populated, etc., did you know that the 360/20 did not have an adder?
This made me
curious about how primitive it was, but the FE docs on
bitsavers show a 16bit ALU that can do ADD, AND, OR and XOR and maybe
some logic to help with BCD math.