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Thank you for this article, I love it. The physical difference between ROM and RAM core memory is not entirely clear to me. If I read it correctly, what causes

Thank you for this article, I love it. The physical difference between ROM and RAM core memory is not entirely clear to me. If I read it correctly, what causes something to read as 1 or 0 depends on whether the wire was wired through or around the core. How does this work for RAM then?

kens 7y ago
The core rope memory (ROM) and erasable core memory (RAM) are different in most ways. For RAM, a core is magnetized in one direction for a 1 and the other direction for a 0. Reading the core erases it, so it must be rewritten (destructive read). On the other hand, for rope the wiring determines the value of the stored bits and it can't be changed. RAM is addressed in an X-Y grid, using core's "coincident current" property. An X line gets half the current necessary to flip a core, and a Y line gets half the current, so only the core where both cross gets flipped. When writing a core, the core is flipped to the 1 state unless an inhibit line blocks it, causing it to stay in the 0 state. Core rope, on the other hand, uses inhibit lines for addressing, blocking all cores except the desired one. Another key difference is RAM stores one bit per core, while rope (ROM) stores 192 bits per core, making it much more dense. (Each rope core has 192 sense wires passing either through or around it.) RAM uses tiny ferrite cores, while rope uses much larger cores consisting of a permalloy ribbon wound around a bobbin. RAM uses a separate plane of cores for each bit in the word, while all the bits of a word share the same core in a rope. Both RAM and rope use sense wires threaded through the cores, along with sense amplifiers to pick up the weak signals. I've written about the AGC's RAM core memory here: http://www.righto.com/2019/01/inside-apollo-guidance-computers-core.html http://www.righto.com/2019/01/inside-apollo-guidance-compute...

kens 7y ago

The core rope memory (ROM) and erasable core memory (RAM) are different in most ways. For RAM, a core is magnetized in one direction for a 1 and the other direction for a 0. Reading the core erases it, so it must be rewritten (destructive read). On the other hand, for rope the wiring determines the value of the stored bits and it can't be changed. RAM is addressed in an X-Y grid, using core's "coincident current" property. An X line gets half the current necessary to flip a core, and a Y line gets half the current, so only the core where both cross gets flipped. When writing a core, the core is flipped to the 1 state unless an inhibit line blocks it, causing it to stay in the 0 state. Core rope, on the other hand, uses inhibit lines for addressing, blocking all cores except the desired one. Another key difference is RAM stores one bit per core, while rope (ROM) stores 192 bits per core, making it much more dense. (Each rope core has 192 sense wires passing either through or around it.) RAM uses tiny ferrite cores, while rope uses much larger cores consisting of a permalloy ribbon wound around a bobbin. RAM uses a separate plane of cores for each bit in the word, while all the bits of a word share the same core in a rope. Both RAM and rope use sense wires threaded through the cores, along with sense amplifiers to pick up the weak signals. I've written about the AGC's RAM core memory here: http://www.righto.com/2019/01/inside-apollo-guidance-computers-core.html http://www.righto.com/2019/01/inside-apollo-guidance-compute...