> The Mod 40 (and probably higher models) had a 1401 simulator. > You executed the simulator (an s/360 program. I don't remember > what operating system) and it would read and process the 1401 > programs. I have no idea haow this was accomplished. There > very well may have been microcode assists of some type.
Heh. It might have been mine. 1974? My employer was approached by a company that had a legal requirement to modify its 1401 code, but they'd lost the source. It's tricky because the 1401 boot process sets the word marks and thus defines instruction boundaries. I056 comes to mind - Read a card (into location zero) and branch to location 56 - the back of the card conatins the instructions to move the data to storage and set the word marks that give it meaning. IBM object code only used a small subset of the 1401's instructions, so there were only a few to "emulate". I used the first six bits of the /360 byte for the six bits of a 1401 BCD character, the seventh for a word mark and the eighth for a group mark. Then once the object code was loaded, I disassembled it back to Autocoder. All went well until we found a patched deck in which someone had used an instruction outside the above set. So I had to write support for another instruction. And another. A few weeks later, between engagements, the SEO asked me to complete the support. After I left the company, they built a complete execution environment on top of my emulation, which was until I left just for the purpose of loading the code to disassemble it. Most of it was written on a train - the 09:00 from Kettering to St Pancras, sitting in a reverse-marshalled half guard's van right at the back of the train, because the tables were double width and I could spread out. It's an axiom of system programmers' lives that you can get a HELL of a lot done if the phone's not ringing. -- Phil Payne http://www.isham-research.co.uk +44 7833 654 800 ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
