On 2022-04-21 00:19, Seymour J Metz wrote:
That has at least two bugs: the first test will incorrectly treat 1*-1
The task is to form the product of two POSITIVE integers.
as having an overflow and the second test is testing all of R0,
The second test must test R1, as shown, not R0.
not just the high bit.
________________________________________
From: IBM Mainframe Assembler List [ASSEMBLER-LIST@LISTSERV.UGA.EDU]
on behalf of Ian Worthington
[00000c9b78d54aea-dmarc-requ...@listserv.uga.edu]
Sent: Wednesday, April 20, 2022 9:46 AM
To: ASSEMBLER-LIST@LISTSERV.UGA.EDU
Subject: Detection of integer overflow
Whilst looking at reliable techniques to detect signed and unsigned
overflow in integer multiplication I was checking out the late John
Erhman's "Assembler Language Programming for IBM System z™ Servers" in
which I discovered he presented this problem and solution:
18.2.13.(2)+ A programmer wanted to test whether the product of two
positive 32-bit binary
integers was too large to fit in a 32-bit register. <snip>
Consider multiplying 75141×56789: the product X'FE5808A9' is indeed 32
bits long but appears to be negative, −27785047. An additional test is
needed:
L 1,X Load first operand
M 0,Y Multiply by second operand
LTR 0,0 Check high-order 32 bits
BNZ NotOK If not zero, product is too big
LTR 1,1 Check high-order bit of GR1
BZ ProdOK Branch if high-order 33 bits are 0s
- - - Not OK
X DC F'75141'
Y DC F'56789'
One hesitates to suggest it, but surely this cannot be correct? This
checks that r0 and r1 are both zero. Surely John meant BNM ProdOk as
the final instruction (at least for signed 32 bit integers: no
further test is required for unsigned integers, I think.)?
Google finds no errata for John's book. It is, of course, much more
likely I've misinterpreted something that John made an error!
