1620 owner here. Sure there are ways to cause a CHECK STOP, and one-instruction infinite loops, including the IBM-sanctioned one you describe below - which everyone used all day to clobber core - but that's all they are, they don't damage hardware.
You can make an infinite loop that is sort of less than one instruction. It's an instruction with a self-referencing indirect address. (Only applies to Model II, and Model I machines with the Indirect Addressing feature installed, like mine.) When it tries to fetch the indirect operand, it loops in the Fetch cycle without ever reaching Execute stage. While that sounds like a core hammer, it is still less than 30% duty cycle on any one address. Also remember the core stack has air blowing through it nonstop. There is a setup on the Customer Engineer switch panel (INC plus BYP) that will hammer a single address, but I don't remember a warning. The only damage risk is if one or more of your decode switch transistors are failing to ground their A/B/C/D drive line, or you have an open X or Y line. In that case there's an inductive voltage spike that will stress all the other D-S transistors. The CE Reference Manual warns against clocking in this state for more than a couple seconds. Dave Wise in Hillsboro Oregon The entire first production run (Suffix B) was paper tape only. Card support was added at Suffix C. The core memory array is 3D with two-level selection. The main array select lines are X and Y. They are driven by 10x10 matrix switches which are in turn driven by 10 decode switches each on A, B, C, and D. A and B select X, C and D select Y. This is slow but it minimizes the number of costly high-performance transistors. The 1401 is the same. ________________________________ From: CAREY SCHUG via cctalk <cctalk@classiccmp.org> Sent: Sunday, November 3, 2024 7:13 AM To: General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> Cc: CAREY SCHUG <sqrfolk...@comcast.net> Subject: [cctalk] HCF [was: System 360 question] I'm going to suggest that the 1620 had the most HCF instructions of any commercially produced computer, ever. And the most intentionally used on a daily basis. It is a memory-to-memory computer with no actual registers (the model II had index registers, but they were in memory). Few machines had disks, and fewer had tape (magnetic or paper), so they were almost always run from tabulating cards. I always started my session with the clear memory command below, and maybe several more times during my scheduled hour. 1. you could press "insert" on the typewriter and enter a program starting at location zero which was executed when you hit return. If you wanted to clear memory, you typed in 12 digits "310000900010" (transmit record) which meant copy the character at location 9 to location 10, then 10 to 11, etc until it found the special character "record mark", which it would never find as you typed in a zero at the starting address. watch the address lights till it looped and hit reset. Or "TF 9,8" (transmit field) which worked the other direction looking for a field mark, which it would never find either. There was a two card program you could put in front of your card deck that would zero memory then load your program, but since the model 1 did addition by table lookup in locations 100-199, if there was any chance that table was bad, it would not work. I think other "catch fire" instruction variants would continue forever. 2. remember that record mark? if you ever executed an instruction with a record mark in the address, you got a MAR check red light, a hard reset was required to escape. probably if in the op code also. And some hard stop for any invalid op code, but these may have been in the category below. 3. there were other errors, but you could press start to continue and there was a toggle switch to ignore at least some of them, like a parity error in memory. 4. I seem to recall being told "don't do this, it can break the hardware", but don't remember any details, and not an instant thing, it would be like the "B *" loop, causing issues if left running a long time. The IBM 1401 had a similar architecture (but with 7 useable bits per address vs 5 for the 1620, and was also memory to memory, so probably had instructions that would run forever. No typewriter available on the 1401 AFAIK, so my example would not have been a routine action.. <pre>--Carey</pre>
