On Sep 10, 2010, at 00:53, robin wrote:
> From: "Paul Gilmartin" <paulgboul...@aim.com>
> Sent: Thursday, 9 September 2010 4:32 PM
>
>> I would firmly oppose such a gentle correction. Take a broad view.
>> Which has the more obvious meaning:
>>
>> LA R3,-4(R2) Set R3 to 4 less than R2
>
> It doesn't.
> It sums 4092 and the content of R2.
> If the content of R2 is positive, the sum is larger than the original content
> of R2 by 4092.*
> If the content of R2 is negative, its value is increased by 4096.
> However, if the intermediate sum were negative, the value being
> deposited in R3 [being truncated to 24 bits) would become a large positive
> value.
> In all cases, R3 becomes larger than R2.
> [I'm assuming that you meant to write LA R3,-4(0,R2) ]
>
I did not. As I expanded a few hours later, in a SYSPRINT excerpt:
R:1 FFF000 2 USING -4096,R1
000000 4132 1FFC FFFFFC 3 LA R3,-4(R2)
The assembler observes there is a USING that allows resolution
of the coded address, -4, and generates an instruction which
adds:
4092 The resolved displacement
-4096 From R1, the resolved base
(R2) Specified in the instruction
Setting R3 to 4 less than R2 (modulo address range, as the
hardware always operates).
> [* The resultant sum must not exceed 24 bits, of course in classic
> base-displacement addressing.]
>
or 31 in neoclassic. The value is stored modulo that address
range; the condition code is not set; no exception is taken.
If the initial content of R2 is less than 4, R3 may be set to
a large positive number. If the initial content of R2 is
outside address range, R3 may be set to far less than (R2)-4,
always correctly modulo address range. This is the way the
address generation hardware has operated from s/360 onward.
>> or:
>>
>> LA R3,4092(R1,R2) Set R3 to 4 less than R2
>
> It doesn't either [viz, set R3 to 4 less than R2].
> It sums the contents of Reg 1 and Reg 2 and the displacement value 4092.
>
You deleted the context. R1 was loaded with -4096.
>> ? The first hardly needs the comment to show the programmer's
>> intention,
>
> What the first comment does is to show the reader that the programmer
> made a programming error.
>
The assembler discreetly refrains from making guesses concerning
the programmer's intention.
> It's the same for the second LA, where the programmer has made two
> erroneous assertions.
If you're referring to "LA R3,-4(,R2)" or "LA R3,-4(0,R2)", agreed.
I made a coding error which the assembler should have reported but
failed to.
-- gil
