That has at least two bugs: the first test will incorrectly treat 1*-1 as having an overflow and the second test is testing all of R0, not just the high bit.
-- Shmuel (Seymour J.) Metz http://mason.gmu.edu/~smetz3 ________________________________________ 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! Best wishes / Mejores deseos / Meilleurs vœux Ian ...
