Cheapest type of Read-Only Memory allowing Random access before Year 1970

Question

Back in the 50s and 60s people had:

• random access, read-write memories like Magnetic Cores.
• sequential access, read-write memories like Delay Lines and Magnetic Tapes.

And:

• sequential access, write once, read-only memories like Punch Card and Perforated Tape, which are not called ROM traditionally.

So what about random access, read-only memories which are called ROM now? Which kind of random access, read-only memories, say Core Rope Memory, Mask ROM, PROM, TROS, CROS..., was cheapest per kbit before Year 1970? And which kind has the simplest decoding/addressing mechanism?

PS:

• Historical price of ROM talks about solid state ROM, which is totally different a question.

• By saying random/sequential, there lies Direct-access storage device (DASD, like Magnetic Drums and Data Cells) in between, which has fixed address and variable access time, while this question talks about ROMs without delay of seeking.

Answer 1

The question seams to imply that there was a need for a ROM in the same sense as it was used in the 1970s and 80s, but why wasting address space and money on storage that could only be used for one purpose?

Machines, way passed the mentioned cut of date of 1970, used magnetic media (drums) and magnetic core for RAM. Both are persistent storage, this means information stored there will be present even after a power cycle. All a CPU needed to do is starting to operate from that memory. Data processing and scientific computing relied on having RAM for changing tasks, while process automation as well was happy with loading core once a while and after that simply used it ROM like - some machines had even ways to switch of writing of (new) data, essentially turning core into ROM.

Long story short: Prior to the 1970s there was no need for ROMs in the sense we know today.

Applications of ROM like storage was, if at all, needed in micro program storage, but solutions here were rather specific to each CPU manufacturer and machine, like CCROS, TROS and similar (*1,*2). Also, noone really wanted ROMS. In fact, this is the very reason why IBM developed the floppy drive: to load the microcode from a persistent media to micro code storage. That way they could use RAM,but kept the content static - and change it when needed.

All ROM like solutions of the time were special to type and hand made.

ROMs as we know now grew out of decoders. Fixed function devices translating information - like turning a key press into ASCII. A job that for early terminals was done literally with piles of diodes (*3). But even this use did only come up after 1970. As only then the need for high volume production, i.e. more than a few dozend, became a thing.

was cheapest per kbit before Year 1970?

Since there were no common, interchangeable solution before 1970, but only partitial solutions specific to each device, comparable prices can not be given.

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*1 - I remember a TROSS like system using modified punch cards instead of special mylar sheets. While not as dense, they had the advantage that ROM content could created by using a standard key punch. Thus patching a ROM was simply DUPing the card until the change, enter the new value and DUPing again until the card was done. ROM changes within minutes, needing only equipment available in any computing center - beside the special cards that is. Even more, a new ROM stack could be delivered as tape and outputted by using a standard card punch.

Depending on one's POV, this may qualify as 'cheapest', still, it's a low volume hand made thing.

*2 - Core Rope Memory was as well a very specific niche solution

*3 - I remember that in 1977 I got hold of two scrapped terminal keyboards, which served as my main source for diodes for several years, as every key was connected to a row of 0..7 diodes to encode its ASCII value.

Answer 2

Here are two "random access, read-only memory" solutions as an example on how those were used on early computers. Both map into the address space of the CPU, and are intended to use for bootstrap programs, that load the core memory with the actual program (that one would today have in ROM).

That worked because core memory retains its contents even after power loss, so you can basically use it as a sort of ROM. It only needs to be loaded once, or re-loaded in case it gets overwritten, or has a bit-flip or similar.

The CDC series (and the Cray, too, I think) had a deadstart panel. Here is a picture of the deadstart panel of a CDC 6600, and this link has a pictore of a more fancy one, with a description. It was essentially a set of 12 x 12 switches (so, 144 bits), and would contain the bootstrap program for a peripheral processor (PP), which then would first initialize itself from some other kind of storage, and then the main CPU.

The PDP-8/E had a MI8-E Bootstrap Loader card. I haven't been able to find a full picture, but this link has some close-ups. It's essentially 32 words of 12 bits (344 bits in total) hard-coded with diodes, with different variants for different bootstrap programs for different devices. You could set the start and load address with wires on the cards.

I have not found any prices for those, but from the low number of bits, you can see that it was not economical to attempt any kind of "ROM". The PDP-8 price list from 1967 lists 4 kWords (with 12 bits) of core memory at around $8000 dollars. While it was technically possible to make core rope memory at the same density, I'd would have expected it to be much more expensive, as it was no longer regular, and each bit would have to be painstakingly programmed and checked manually.

Answer 3

EDSAC (operational in 1949) had read-only memory to hold its Initial Orders. The Initial Orders were wired on to uniselectors, and when running, allowed the easy (!) loading of other programs from paper tape. That is, they provided the services of a boot ROM, with some aspects of an assembler built it.

Since this was post-war Britain, the uniselectors were almost certainly surplus equipment, and therefore cheap.

The capacity was around 50 words; see this link for a listing of "Initial Orders 2". There's a picture of the uniselectors on a modern reconstruction of EDSAC here.

Answer 4

The Apollo Guidance Computer used Core Rope memory. The read-only instructions were woven into the rope (in one story, by canonical little-old-ladies)

Before that, computers used diode matrix memory, and switched relay selection of plugboard settings, both of which were fixed access time read-only data stores.