On Thu, 29 Nov 2012 11:09:39 -0600, Paul Gilmartin wrote: >(The worst case was the 6502, which ignored carries out of the low half >of the address, at least sometimes.)
No, it didn't. You are talking about the conditional branch instructions. The branch offset is an 8-bit signed integer. When the branch is to be taken, the offset is added to the program counter. That is done in one cycle, at the end of which the resulting address is put on the address bus. If there is a carry, the data returned is ignored. The carry is added to the high byte of the address and the corrected address is placed on the address bus during the next cycle. The carry is not ignored. In the 6505, every clock cycle was a memory cycle. IIRC, this was the only case when a memory reference was ignored. -- Tom Marchant ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
