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The TX-0 was a transistor computer (reportedly the first ever built), at the MIT Lincoln laboratory. It was a predecessor to the TX-2, and an influence on the design of the PDP-1. It was in some sense a successor to the Memory Test Computer, itself a spin-off of the pioneering Whirlwind.

For I/O, it had a Flexowriter, a paper tape reader and punch, and a display CRT. The front panel, in addition to the usual operations, had an array of switches which allowed any combination of the bottom 16 main memory locations to be substituted by a line of switches.

It had a word size of 18 bits, and its only generally program-accessible register was the accumulator. The first two bits of the instruction were an operation code (yes, it only has 4 basic instructions); the remaining 16 bits were an address (or specified the action selected for the "operate" instruction). The instructions were:

Opcode Instruction Function
00 STO Store accumulator contents in memory
01 ADD Add contents of memory to accumulator
10 TRN If accumulator is negative, jump
11 OPR See table below

Values (in octal) for the 'address' field in the 'operate' instruction are:

Value Name Action
0100000 CLL Clear left half of accumulator
040000 CLR Clear right half of accumulator
030000 Hlt Halt machine
020000 IOS 'In-Out Stop' - pause machine for I/O operation
07000 PRH Punch 6 data holes from accumulator, and the 7th hole
06000 P6H Punch 6 data holes from accumulator
04000 PNT Print Flexowriter character from accumulator
03000 R3C Read 3 characters from Flexowriter into accumulator (shifted between characters)
02000 DIS Intensify a point specified by the accumulator on the display scope
01000 R1C Read character from Flexowriter into accumulator
0600 CYR Rotate the accumulator right one bit
0400 SHR Shift the accumulator right one bit
0200 MLR Store the contents of the MBR (below) in the LR (below)
0100 PEN Read the light pen flip-flops into the accumulator
040 COM Complement the accumulator
020 PAD Half-add (XOR) the MBR into the accumulator
010 CRY Half-add the implied carry into the accumulator
04 TAC OR the TAC (below) into the accumulator
03 TBR Store the TBR (below) in the MBR (below)
02 LMB Store the LR (below) in the MBR
01 AMB Store the accumulator in the MBR

Many of these can be set together, and perform useful operations. E.g. as a simple example, CLL+CLR clears the accumulator; a more complex one is that AMB+PAD+CRY does a left rotate.


These are the registers (not all accessible under program control) in the original design. Later updates changed some register widths and added registers.

Name Size (bits) Meaning
AC 18 Accumulator
MBR 18 Memory buffer register
MAR 16 Memory address register
PC 16 Program counter
IR 2 Instruction register
LR 18 Live register
TBR 18 Toggle switch buffer register
TAC 18 Toggle switch accumulator

Test mode

The console has a switch to set test mode in which operations are taken from the TBR. The operations are similar to regular instructions, but with a different interpretation:

TBR 0-1 Instruction Meaning
00 STO x Store TAC in memory location x.
01 ADD x Add memory location x to AC. (Check MBR to examine.)
10 TRN x Change to normal mode and start running at location x.
11 OPR x Execute the instruction.

External links