I just look at it from this view:
Normally, 'C' in an instruction mnemonic indicates character actions,
not numeric actions. 'P' indicated packed decimal actions. And, the
absence of such indicated the default numeric actions.
(At least, before all the new stuff.)
So, for numeric, it was 'L' was a fullword, and 'LH' is a halfword' and
'LB' for a byte. It could just as easily been Load Numeric ('LN', 'LHN'
and 'LBN') but the original designers did not go that way. Maybe for
less keystrokes on punch cards because these were the more common
instructions used?
Tony Thigpen
David Cole wrote on 8/6/25 3:18 PM:
I apologize is my initial post was unclear. My question is about IBM's
choice of instruction names, not about functionality.
Also, this has nothing whatsoever to do with the C language.
Let me edit my post and re-present it here...
In Principles, these two machine instructions are presented:
- LB loads a byte into a register and sign-extends it.
- LLC also loads a byte into a register but then zero pads it.
That's all pretty clear, but...
My question is, why did IBM choose have the instruction's name end with
B in one case and with C in the other?
- On the one hand, Why didn't they choose LC (instead of LB) as
that instruction's name?
- Or on the other, why didn't they choose LLB (instead of LLC) as
that instruction's name?
Just curious.
Dave Cole
At 8/6/2025 05:42 AM, David Cole wrote:
In Principles:
- LB loads a byte into a register and sign-extends it.
- LLC also loads a byte into a register but then zero pads it.
That's all pretty clear, but...
Why use B in one case and C in the other?
- Why not LC instead of LB?
- Or why not LLB instead of LLC?
"Inquiring minds what to know."
Dave Cole
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