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Results tagged with instruction-set
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user 148
For questions regarding the instruction sets of microprocessors.
2
votes
Is there a CPU ISA preferring a test for the value of one over testing for zero?
The Burroughs B5000 has no way to test a complete word directly against a value, be it zero or one. Looking at the instructions (section 5), the only conditional branch operations are BFC (branch forw …
- 30.5k
6
votes
Accepted
In what way does the Straight-8 expand on the PDP-5?
There is a section in the DEC FAQ that describes the differences between the PDP-5 and PDP-8 instruction set:
Compatability: The core of the PDP-8 instruction set is present, but
memory loc …
- 30.5k
45
votes
What motivated the weird boolean instruction repertoire of the PDP-11?
What am I missing?
You are missing that in really the majority of real-world cases, and, or and not are used for bitmasks. And you need bitmasks all the time, in particular if you don't have that mu …
- 30.5k
4
votes
Is there a comprehensive description of the Electrologica X1/X8 instruction set?
I can give you my tables for the X1, if that helps. I didn't keep track from where I compiled them exactly. Looking over it again, most seems to come from Dijkstra's PhD Thesis [1].
The binary encodin …
- 30.5k
50
votes
Accepted
Why did the PDP-11 include a JMP instruction?
Besides the flags, and differences in cycle count, the more important difference is that JMP x uses the effective address of x, while MOV x,R7 uses the value at x. In other words, there's one level le …
- 30.5k
5
votes
Which CPUs had instructions leaving data registers in an unspecified state?
One example are the two undocumented 8085 flags that can be found by reverse-engineering the silicon (well, it's part of a register, not a complete register). They have uses for signed overflow in ari …
- 30.5k
11
votes
Origin of the NZVC condition codes scheme?
My take is that this scheme has been invented the same way everything has been invented: Slowly, by evolution and combining existing ideas, and not "from scratch" by a strike of genius.
The carry fla …
- 30.5k
25
votes
Executable ASCII files before x86?
If you go back a lot before the x86, this technique wasn't unusual at all. In fact, writing programs using printable letters and symbols was pretty much the norm for early computers, except that there …
- 30.5k
5
votes
Origin of "arithmetic" and "logical" for signed and unsigned shifts
(Another "what was the first" question where it's basically impossible to answer it unless one goes through all computer instruction sets ...)
One example of the usage of "logical" is the IBM 7090 (19 …
- 30.5k
10
votes
Accepted
What is the history of the PDP-11 MARK instruction?
Partial answer:
If you compare the PDP-11 calling convention using a stack as described e.g. here
! parameters passed on stack
...
mov parm1,-(sp)
mov parm2,-(sp)
…
- 30.5k