Memory Test Computer

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The Memory Test Computer was an early vacuum tube computer which was a spin-off of the pioneering Whirlwind effort; it was built by Ken Olsen and others to test the first core memory, devised for Whirlwind. It was apparently the second computer built at MIT.

It was a parallel computer, with a 16-bit word length (since the memory it was to test was for Whirlwind, which was that size); it was similar in many other ways to Whirlwind, also.

It first ran in March, 1953; the prototype core memory was installed and operational by the end of May. Later, the MTC was used to test various system components to be used in the SAGE system. It was shut down permanently on March 21, 1958; but unlike Whirlwind and the later TX-0, it unfortunately was apparently not preserved. It was a considerable influence on the TX-0, through people who had worked on the MTC.

Technical details

The MTC had 26 instructions; for main memory it had 4KW of core memory (after the main core memory unit had been moved to Whirlwind), a drum (originally only 4KW in size), and 64W of ROM, implemented in a toggle switch array and a plug-board. (NB: memory locations were referred to as "registers" in the documentation.) Addresses consisted of a 4-bit 'field number' and an 11-bit 'address' within that field; field 0 was the ROM, 1 and 2 were the core, and 3 and 4 were the drum. The pair of a field number and an address were termed an 'extended address'.

Most registers and data paths had an additional bit, for parity error checking. Bits were numbered using big-endian notation; the MTC used one's complement arithmetic (overflows caused an 'overflow alarm', which halted the MTC). For I/O it had two paper tape readers and a punch, printers, and several displays (one with a camera controlled by the MTC).


There were 11 visible registers (built out of flip-flops) in the final design:

Function Name Size (bits) Use Comment
PC   11 Program Counter
Partial Sum   16? General purpose Together termed the 'Accumulator (AC)', but that name was often informally applied to the Partial Sum Register
Carry   1? Momentarily store carries generated in addition operations
A AR 16 Buffer register between all the data sources and sinks in the system; connected to 'A-Register-In' and 'A-Register-Out' buses
B BR 16 Effectively an extension of the PSR during multiplications
Live Register 1 LR1 16 Can be substituted for any ROM location Addressed as main memory in field 0
Live Register 2 LR2 16 Can be substituted for any plug-board location
Live Register 5 LR5 16
Live Register 3 LR3? 16
Live Register 4 LR4? 16
Operation Field Register   4? Can only be written, not read
Program Field Register   4? Loading does not take immediate effect


Instruction words contained a 5-bit operation code, and 11 bits of address - although in some instructions the 'address' field is used for other purposes. Since the address portion was not long enough to hold an extended address, the MTC used bank switching to gain access to all of main memory.

Opcode Abbreviation Function Comment
0 ha Halt machine
1 mh Multiply AC by memory, leave result in BR,AC
2 ds Display memory contents
3 ic 'Identity check'; compares AC with memory contents
4 st Store AC in memory
6 ra 'Replace address'; stores bits 5-15 of AC in memory
7 rf 'Return from'; stores bits 5-15 of LR2 in memory
8 ca 'Clear and add'; load AC from memory
9 ad Add memory to AC
10 cs 'Clear and subtract'; load AC with negative of memory
11 su Subtract memory from AC
12 et 'Extract'; AND memory into AC
14 ch 'Charactron display' Refer to Programming Manual
15 sm 'Subtract Magnitudes'; copy AC to BR, subtract memory from AC
16 tr 'Transfer'; load memory address into PC Consult Programming Manual for side-effects on LR2
17 tro 'Transfer Out'; load memory address into PC, update field of PC from Program Field Register
18 tn 'Transfer on negative'; if AC is negative, as 'tr' above
19 tno 'Transfer on negative out'; if AC is negative, as 'tro' above
20 md 'Memory address display' Refer to Programming Manual
21 sof 'Select operation field'; load Operation Field Register
22 to 'Transfer on overflow'; if an overflow condition exists, as 'tr' above
24 cr 'Cycle right' Refer to Programming Manual
25 sr 'Shift right'
28 pr 'Print/punch'
30 ri 'Read in'
31 op 'Operate'

External links