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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Apollo 11’s Lunar Module Guidance Computer had 2,048 words of erasable memory—the writable, RAM-like memory used for changing data. That works out to 4,096 bytes (4 KiB) when the stored parity bit is counted, or 3,840 bytes of data bits alone. The computer also had 36,864 words of separate fixed memory, which was not RAM.
What does “2,048 words” mean in bytes?
The Lunar Module computer stored memory in words, not in the byte-addressed units familiar from modern PCs. NASA’s technical report describes each word as 16 bits: 15 data bits plus one odd-parity bit. So 2,048 words contain 32,768 stored bits, or 4,096 bytes (4 KiB) if parity is included. Counting only the 15 data bits in each word gives 3,840 bytes of payload.
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“About 4 KB of RAM” is therefore a useful shorthand, but it should not be mistaken for a claim that the computer used a modern byte-addressed RAM layout. NASA’s technical report on the Apollo Guidance Computer specifies the word format.
How was erasable memory different from fixed memory?
The Lunar Module computer had two distinct memory types. Erasable memory held values that changed during operation, such as variables and counters. Fixed memory was wired core-rope memory used largely for executable programs and constants. It is broadly comparable to ROM, not additional RAM.
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| Memory type | Capacity | Role | Modern analogy |
|---|---|---|---|
| Erasable memory | 2,048 words | Writable working data, including changing values | RAM |
| Fixed memory | 36,864 words | Mostly fixed programs and constants | ROM |
NASA’s Apollo computer references give these capacities; Peter Adler, a software contributor at MIT Instrumentation Laboratory, likewise described 2,048 words of erasable memory and 36,864 words of fixed memory in the Apollo 11 Lunar Surface Journal account of the program alarms.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Did the 1201 and 1202 alarms mean the RAM was full?
No. During lunar descent, the computer displayed 1201 and 1202 program alarms. NASA’s mission summary attributes the overload to extraneous rendezvous-radar data and explains that the computer continued high-priority work. That describes an overloaded computer managing task priorities; it does not establish that all erasable memory was full or that the alarms were a simple equivalent of a modern “out of memory” error. See NASA’s Apollo 11 mission account.
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