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The [[instruction set|instruction format]] was 3 bits for opcode, 12 bits for address. Block I had 11 instructions: TC, CCS, INDEX, XCH, CS, TS, AD, MASK, SU, MP, and DV. The lower 8 instructions were directly accessed by the 3 bit op code. The upper 3 instructions were accessed by performing a special type of INDEX instruction (called EXTEND) immediately before the instruction.
The AGC computer had 5 vectored interrupts. One interrupt (DSRUPT) was reserved for updating the user display (DSKY). Another interrupt (KEYRUPT) signaled a keypress from the user's keyboard. Another interrupt (T3RUPT) updated the AGC's real-time clock.
The user interface was an array of numerals and a calculator-style keyboard (electronic calculators were unknown at the time - slide rules were used). Commands were entered numerically as two digit "prog", "verb" and "noun" numbers. The numerals were green high-voltage fluorescent displays arranged in an array of seven segments per numeral to display numbers. The segments were driven by electromechanical relays, which limited the display update rate. Three 5-digit numbers could be simultaneously displayed. Input was by pushbuttons. This "calculator-style" interface was the first of its kind, the prototype for all similar interfaces.▼
▲The user interface was called the DSKY (display/keyboard); an array of numerals and a calculator-style keyboard (electronic calculators were unknown at the time - slide rules were used). Commands were entered numerically as two digit "prog", "verb" and "noun" numbers. The numerals were green high-voltage fluorescent displays arranged in an array of seven segments per numeral to display numbers. The segments were driven by electromechanical relays, which limited the display update rate. Three 5-digit numbers could be simultaneously displayed. Input was by pushbuttons. This "calculator-style" interface was the first of its kind, the prototype for all similar interfaces.
It's not widely known, but the system failed during the first lunar descent, with a "1201 alarm" and "1202 alarm" (tasks not finished) because the terminal approach (docking) radar was left on and unexpected radar returns from the moon overloaded the CPU. This almost caused a mission abort. The software failed safe, shedding unnecessary tasks, and the inertial guidance tasks continued to operate reliably.
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