21 Nov
2016
21 Nov
'16
6:28 p.m.
On Sun, Nov 20, 2016 at 7:29 PM, Jon Elson <elson at pico-systems.com> wrote:
On 11/20/2016 05:30 PM, Charles Dickman wrote:
I was looking here:
http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/fe/2020/Z33-1021-0…
(pdf) pages 45, 46 describe the ALU
And here:
http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/fe/2020/SY33-1024-…
(pdf) pages 15 and 38 Data flows and ALU schematic
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.