5 ms·

Fixing the core memory in a vintage IBM 1401 mainframe

89 points by kens 11y ago

wglb 11y ago
Fascinating detailed drilldown. (I haven't heard of the 1401 called Mainframes before, though)
- kens 11y ago
  What would you call the 1401 if not a mainframe? IBM refers to the 1401 as a mainframe: http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/mainframe/ http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/mainfra... http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP1401.html http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainfra... The 1401 restoration team calls them mainframes: http://ibm-1401.info/ http://ibm-1401.info/ But whatever you call it, the 1401 is an interesting computer.
  - wglb 11y ago
    Yes, quite. I am wondering if the mainframe tag is a more recent thing, as I don't remember that description back when it was still in production.
    - ChuckMcM 11y ago
      Interesting that it doesn't show up in any of IBM's literature until the S/360 which is referred to as a "mainframe". Most people I've talked with think of the 701 as the first mainframe computer.
Animats 11y ago
I really need to go watch that machine run. They finally got four tape drives working, so now they can run a sort. The 1401 is a strange machine. It's a direct descendant of IBM's electromechanical card machines, and it shows. It has a memory layout and ALU designed for fixed length strings of decimal digits of arbitrary length. The end of a number is indicated by a "word mark" bit, a special 7th memory bit on each 6 bit character. In early computers, "business" computers had decimal arithmetic hardware, and "scientific" computers had binary. The IBM System/360 had both. This lives on in the binary-coded-decimal instructions in x86, which are used by almost nobody. (You can only do one digit at a time, because the x86 decimal instructions haven't been upgraded since x86 machines really were 8 bit.)
- kens 11y ago
  The arbitrary length numbers in the 1401 are interesting because they really are arbitrary length. For instance you can work with 1000 digit numbers if you want. I made two 1000-digit numbers and multiplied them with a single instruction. The console lights on the 1401 flashed like crazy while the hardware multiplier chugged away. A full minute later it had the answer. Yes, a single instruction took a minute to run.
  - eru 11y ago
    That's an instruction-frequency of 0.017 Hz.
- bjackman 11y ago
  If they were 8-bit, shouldn't they have supported 2 digits at at time (4 bits per digit)?
  - Animats 11y ago
    6 bits of data, one bit of mark, and 1 bit of parity only visible to the parity checking hardware.
    - mikepavone 11y ago
      That's not the format used by the x86 BCD instructions. x86 (and the 8080 before it) use 4-bit packed BCD so each byte contains two digits (x86 also has instructions for dealing with unpacked BCD, but the digits are still 4-bit). Additionally, the 8086 was a 16-bit processor (the 8088 had an 8-bit bus, but that came later and was still 16-bit internally) so I don't think the 8-bit limitation is due to the word size of the CPU in this case. It could be a holdover from the 8080 (8086 isn't binary compatible, but it was marketed as "source" compatible so there's a fair amount of similarity), but the 68000 (16-bit implementation of a 32-bit ISA) has the same limitation. I think it probably comes down to cost. Just to support two packed digits, you need a fair bit of extra hardware (or a little extra hardware and a bunch of microcode) so you can properly calculate the correction factor. Expanding this to a full word means you need to be able to do that calculation for 4 nibbles instead of 2 which is about 3 times worse (you effectively get the most significant nibble "for free" so it's 3 extra nibbles to support instead of just 1).
rasz_pl 11y ago
Here is a blog of a guy trying to build core memory from scratch for fun: Weekend Project: A Tiny Core Memory Array: http://mikesmods.com/mm-wp/?p=556 http://mikesmods.com/mm-wp/?p=556 Weekend Project: Determining Operating Parameters of a Memory Core: http://mikesmods.com/mm-wp/?p=610 http://mikesmods.com/mm-wp/?p=610

89 points by kens 11y ago

wglb 11y ago

Fascinating detailed drilldown. (I haven't heard of the 1401 called Mainframes before, though)

kens 11y ago
What would you call the 1401 if not a mainframe? IBM refers to the 1401 as a mainframe: http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/mainframe/ http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/mainfra... http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP1401.html http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainfra... The 1401 restoration team calls them mainframes: http://ibm-1401.info/ http://ibm-1401.info/ But whatever you call it, the 1401 is an interesting computer.
- wglb 11y ago
  Yes, quite. I am wondering if the mainframe tag is a more recent thing, as I don't remember that description back when it was still in production.
  - ChuckMcM 11y ago
    Interesting that it doesn't show up in any of IBM's literature until the S/360 which is referred to as a "mainframe". Most people I've talked with think of the 701 as the first mainframe computer.

Animats 11y ago

I really need to go watch that machine run. They finally got four tape drives working, so now they can run a sort. The 1401 is a strange machine. It's a direct descendant of IBM's electromechanical card machines, and it shows. It has a memory layout and ALU designed for fixed length strings of decimal digits of arbitrary length. The end of a number is indicated by a "word mark" bit, a special 7th memory bit on each 6 bit character. In early computers, "business" computers had decimal arithmetic hardware, and "scientific" computers had binary. The IBM System/360 had both. This lives on in the binary-coded-decimal instructions in x86, which are used by almost nobody. (You can only do one digit at a time, because the x86 decimal instructions haven't been upgraded since x86 machines really were 8 bit.)

kens 11y ago
The arbitrary length numbers in the 1401 are interesting because they really are arbitrary length. For instance you can work with 1000 digit numbers if you want. I made two 1000-digit numbers and multiplied them with a single instruction. The console lights on the 1401 flashed like crazy while the hardware multiplier chugged away. A full minute later it had the answer. Yes, a single instruction took a minute to run.
- eru 11y ago
  That's an instruction-frequency of 0.017 Hz.
bjackman 11y ago
If they were 8-bit, shouldn't they have supported 2 digits at at time (4 bits per digit)?
- Animats 11y ago
  6 bits of data, one bit of mark, and 1 bit of parity only visible to the parity checking hardware.
  - mikepavone 11y ago
    That's not the format used by the x86 BCD instructions. x86 (and the 8080 before it) use 4-bit packed BCD so each byte contains two digits (x86 also has instructions for dealing with unpacked BCD, but the digits are still 4-bit). Additionally, the 8086 was a 16-bit processor (the 8088 had an 8-bit bus, but that came later and was still 16-bit internally) so I don't think the 8-bit limitation is due to the word size of the CPU in this case. It could be a holdover from the 8080 (8086 isn't binary compatible, but it was marketed as "source" compatible so there's a fair amount of similarity), but the 68000 (16-bit implementation of a 32-bit ISA) has the same limitation. I think it probably comes down to cost. Just to support two packed digits, you need a fair bit of extra hardware (or a little extra hardware and a bunch of microcode) so you can properly calculate the correction factor. Expanding this to a full word means you need to be able to do that calculation for 4 nibbles instead of 2 which is about 3 times worse (you effectively get the most significant nibble "for free" so it's 3 extra nibbles to support instead of just 1).

rasz_pl 11y ago

Here is a blog of a guy trying to build core memory from scratch for fun: Weekend Project: A Tiny Core Memory Array: http://mikesmods.com/mm-wp/?p=556 http://mikesmods.com/mm-wp/?p=556 Weekend Project: Determining Operating Parameters of a Memory Core: http://mikesmods.com/mm-wp/?p=610 http://mikesmods.com/mm-wp/?p=610