Hi Warren, I've always tried to avoid x86 and friends for ARM, so I may be wrong, but the run up to the first of the two memcpy() calls looks the same to me. Here's the assembler, values given an RBP of 100, and the stack contents. Good version first, bad second. rbp = 100 L29: movq -8(%rbp),%rax rax = *92 pushq %rax *92 movq 16(%rbp),%rax rax = *116 pushq %rax *92 *116 movq $64,%rax rax = 64 pushq %rax *92 *116 64 movq 32(%rbp),%rax rax = *132 popq %rcx rcx = 64 *92 *116 addq %rcx,%rax rcx = 64+*132 movq (%rax),%rax rax = *(64+*132) pushq %rax *92 *116 *(64+*132) movq $40,%rax rax = 40 pushq %rax *92 *116 *(64+*132) 40 movq 32(%rbp),%rax rax = *132 popq %rcx rcx = 40 *92 *116 *(64+*132) addq %rcx,%rax rax = 40+*132 movq (%rax),%rax rax = *(40+*132) popq %rcx rcx = *(64+*132) *92 *116 addq %rcx,%rax rax = *(64+*132)+*(40+*132) pushq %rax *92 *116 *(64+*132)+*(40+*132) call Cmemcpy rbp = 100 L29: movq -8(%rbp),%r8 r8 = *92 pushq %r8 *92 movq 16(%rbp),%r8 r8 = *116 pushq %r8 *92 *116 movq $64,%r8 r8 = 64 movq 32(%rbp),%r9 r9 = *132 addq %r9,%r8 r8 = *132+64 movq (%r8),%r8 r8 = *(*132+64) movq $40,%r9 r9 = 40 movq 32(%rbp),%r10 r10 = *132 addq %r10,%r9 r9 = *132+40 movq (%r9),%r9 r9 = *(*132+40) addq %r9,%r8 r8 = *(*132+64)+*(*132+40) pushq %r8 *92 *116 *(*132+64)+*(*132+40) call Cmemcpy A glance at the second memcpy() call look equivalent too. So perhaps it's not calculating the parameters to memcpy() that's wrong, but the inputs into those calculations being faulty? I'd use gdb(1) to break at particular instructions, examine memory, etc., to work backwards through the bad version until spotting where good data becomes bad. -- Cheers, Ralph.

On 10/03/19 16:03, Steve Johnson wrote (in part): +42 N. (And sorry, Warren, I haven't written iapx assembler in over a decade.)