Memory Test Computer
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 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 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 are 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 cause an 'overflow alarm', which halts 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).
Registers
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 |
Instructions
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
- MTC - documents at Bitsavers
- M-Series - contains many MTC development memos
- Memory Test Computer Memoranda, 1952-1958
- C. Gordon Bell, Gerald Butler, Robert Gray, John E. Mcnamara, Donald Vonada, and Ronald Wilson, The PDP-1 and Other 18-Bit Computers, in C. Gordon Bell, J. Craig Mudge, John. E. McNamara, Computer Engineering: A DEC View of Hardware Systems Design, Digital Press, Bedford, 1978 - contains some detail about the MTC
- Dick Best and Wally Wheaton with Memory Test Computer at MIT
