The Magnetic drum, the main memory of the USSC90.

                                                               The magnetic-drum-memory (without container). 

While your personal computer maybe equipped with at least 2 GB of direct accessible RAM, 
the USSC90 main memory existed out of a magnetic-drum-memory of only 50kB. 
This was magnetic drum turning around at 295 cycles/Sec. with a capacity 5000 words of 10 digits. 
For each digit five bits were read, 4 bits formed a biquinary coded decimal, 
while the fifth bit was used for parity to enable a hardware check on data integrity.
So the capacity was only 50kB, at least 40 000 times smaller than a personal-computers main memory.
A maximum of 20 words could be fetched during one drum-rotation, 
so the reading or writing happened with the speed of 20*300*10Bytes = 60kBytes/second. 
A personal-computer with a speed to access memory of 400 MB/sec, is 6650 times faster.

The magnetic drum.

The magnetic-drum was concealed in a black-aluminum container, 65 cms high and 50 cms in diameter.
The top-plate contained about 29 connectors, providing electrical access to the motor and to the magnetic heads, 
which read and wrote the data (word by word). 
There was no electronic circuitry inside the container.
The memory was divided into 25 bands with each 200 signed words of 10 digits.
Each band had its own group of five fixed magnetic heads. 
The bands 4000,4200,4400,4600 and 4800 were fast-access bands, since each of them was equipped with four groups of heads, 
each placed 90 degrees apart. For minimum access-time, the CPU-logic selected the heads reading the desired word first.

The I/O-devices automatically wrote/read their information to/from the buffers in a separate part of the drum.  
With one program-instruction this information could be transferred back and forth to main memory, 
to the so-called interlace patterns.

For hardware-purposes there was a timing-band on the drum, from which the addresses were read 
and signals that controlled the buffer-transfers.

Each drum-revolution took 3,4 milliseconds. 
For maximum execution-speed the programmer had to address (=location) the program-instructions corresponding to the rotation of the drum. 

Schematic drawing of the drum.

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