{"id":2060,"date":"2018-03-14T13:05:09","date_gmt":"2018-03-14T13:05:09","guid":{"rendered":"http:\/\/www.sydneysmith.com\/wordpress\/?p=2060"},"modified":"2018-03-14T13:05:09","modified_gmt":"2018-03-14T13:05:09","slug":"the-hp-25-rom-dilemma","status":"publish","type":"post","link":"https:\/\/www.sydneysmith.com\/wordpress\/2060\/the-hp-25-rom-dilemma\/","title":{"rendered":"The HP-25 ROM dilemma"},"content":{"rendered":"

Current thinking has the HP-25 and HP-25C having the same ROMs. After all, they do the same things. It makes a lot of sense.<\/p>\n

This suggests that you can take an emulator that supports continuous memory and add in a standard HP-25 \/ HP-25C ROM. Whilst that does sound like a valid recipe for a HP-25C, it isn’t. There are some issues that may not have been mentioned before.<\/p>\n

The combination doesn’t work for a very simple reason. It doesn’t work because the first thing the ROM does is to clear the memory! No, it doesn’t sound very continuous to me either.<\/p>\n

Clearing memory makes sense on a HP-29C, if it detects a memory lost error. But, as far as I recall, fancy checks like that didn’t come until after the HP-25C. Besides, there doesn’t seem to be any checking beforehand in the HP-25 ROM. It just clears memory regardless. The checks in the HP-29C are pretty obvious. There is nothing like that in the HP-25\/C startup code.<\/p>\n

Now I might be missing something here but, for now, my hypothesis is:<\/p>\n

Despite current thinking, the ROM got upgraded between the models.<\/em><\/p>\n

It wouldn’t need to be much of a change; but it can’t be continuous memory if the first thing you do is forget.<\/p>\n

There is some support for this idea in the chips present in the calculators. It’s not conclusive in what I have to hand but, whilst the listing says certain ROMs were present in both the HP-25 and the HP-25C, it does say that ROM \/ anode driver 1818-0153 was only used in the HP-25. There are plenty of other examples where a chip has “25\/25C” written against it so it is interesting that this one only says “25”. There is also a later chip – the 1818-0168 – that replaces the HP-25 ROM \/ anode driver with a “25\/25C” version.<\/p>\n

The available startup code said to be for a HP-25 \/ HP-25C looks like this:
\n<\/p>\n

\r\nHP25 Logging started.\r\n; S= ...3.5..........\r\n00000 if n\/c goto 00236\r\n00236 c -> addr\r\n00237 clear data registers\r\n...\r\n<\/pre>\n
<\/code>
\nIt is pretty short so there are few opportunities to not clear the registers.<\/p>\n
This is power up code so both the carry flag and the C register are normally zero. If that is the case, then data registers 0-15 are going to get zeroed.<\/p>\n
Your options for avoiding that fate are:<\/p>\n
Option 1<\/strong>. Set carry at “power up” on a HP-25C so the original goto never happens. This might be possible, if the carry flag is available on a CPU chip pin. However, the code that gets run if the goto doesn’t happen (not n\/c), the code from 0001 onward; doesn’t seem to have any relationship to power up code. There is also a “goto 0001” (at 00153) which suggests that the program counter gets to 0001 from there rather than by carry being set at power up. I should note that both the HP-25 and HP-25C are said to use the same CPU chip so it can’t be that the CPU chip behaves differently when placed near continuous memory RAM. It’d have to be a pin and different wiring.<\/p>\n
Option 2<\/strong>. Put some value other than 0 in c at power up if it is a HP-25C. That’d be possible, but again you’d need a CPU pin to make it boot up differently when installed in a HP-25C.<\/p>\n
Short of extreme measures that would be difficult to build into the CPU (having a “clear data registers” opcode only work when the program counter isn’t 00237, or perhaps until after four other instructions have executed); the above are your only two options.<\/p>\n
An extra pin is possible. They would have needed to know beforehand: that “continuous memory” was going to come along, that they were going to upgrade the HP-25, that they would need to change the behaviour of the existing microcode; and that they would do it by forcing carry high or by forcing a value into c.<\/p>\n
The Dilemma<\/h2>\n
How can the same ROM with the same CPU and the same power up conditions produce different outcomes?<\/p>\n
If the output is determinable from the inputs and the inputs are the same and the processes are the same then the outputs should be the same.<\/p>\n
Solutions<\/h2>\n
Solution 1<\/strong>. The most likely explanation is the ROM \/ anode driver got upgraded between the HP-25 and the HP-25C and the ROMs weren’t the same. The new ROM doesn’t clear the data registers on power up. The calculator will start with zeros in the volatile RAM in a HP-25; or with existing values in the “continuous memory” RAM in a HP-25C. The easiest fix would be to replace the “clear data registers” at 00237 with a “nop”. Perhaps that was the change between the two ROM \/ anode driver chips.<\/p>\n
Solution 2<\/strong>. Add an extra input that is different between the two calculators. This is the extra pin solution.<\/p>\n
HP-25 \/ HP-25C Emulator<\/h2>\n
I believe:<\/p>\n