IBM 305 RAMAC - Architecture

Architecture

System architecture was documented in the 305 RAMAC Manual of Operation. The 305 was a character-oriented variable "word" length decimal (BCD) computer with a drum memory rotating at 6000 RPM that held 3200 alphanumeric characters. A core memory buffer of 100 characters was used for temporary storage during data transfers.

Each character was 7 bits, composed of two zone bits ("X" and "O"), four BCD bits for the value of the digit, and an odd parity bit ("R") in the following format:

X O 8 4 2 1 R

Instructions could only be stored on 20 tracks of the drum memory and were fixed length (10 characters), in the following format:

T1 A1 B1 T2 A2 B2 M N P Q
Field positions Function
T1 A1 B1 Source operand address – Track, low order AB character
T2 A2 B2 Destination operand address – Track, low order AB character
M N Length of operands (each operand must be entirely on its specified track)
P Program exit code; used to select test conditions, perform jumps, and initiate input/output. The 305's control panel programming determines the action(s) performed.
Q Control code; modifies the operation (similar to an op code), the default operation being a copy from source to destination. Other operations were: "1" Compare, "2" Field compare, "3" Compare & Field compare, "5" Accumulator reset, "6" Blank transfer test, "7" Compress & Expand, "8" Expand, "9" Compress


Fixed-point data "words" could be any size from one decimal digit up to 100 decimal digits, with the X bit of the least significant digit storing the sign (signed magnitude).

Data records could be any size from one character up to 100 characters.

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