How the Big Blue Grinch Stole the Apollo Guidance Computer … Only They Didn’t! Hugh Blair-Smith MAPLD 2005

Replace MIT’s AGC with IBM’s LVDC?? IBM FSD’s self-image “Established” Boost Guidance Taking sides: Industry vs. Academe TMR for fault tolerance Consultant role of Bellcomm

LVDC Architecture (1) Serial logic –Reduces transistor count –Performance penalty –Core memory access is still parallel –Bits in low-to-high sequence for carry propagation “Word time” is thus determined by carry propagation rate My estimate: 3 word times per memory cycle –Forces 2’s complement notation: logical negation is separate –Separate multiplication subsystem, reminiscent of 1940s 7 (I think) instructions needed to fill time until product is ready

LVDC Architecture (2) Pedestrian instruction set –No exchange-with-memory –No divide –No facility for indexing or indirect addressing –Not easily supportive of multiple precision –Adequate for evaluating Adaptive Polynomials: the “APE” Mediocre at best as a general purpose computer

LVDC Architecture (3) Triple Modular Redundancy (TMR) –Fail-operational rule (FO) –Appropriate for high-risk environment inside booster –Forces serial logic to save triplicated transistor count –No clear route to a degraded fail-safe (FS) state –My guess: this was the clincher for Bellcomm

LVDC Architecture (4) Interfaces –Inputs: IMU, ground commands, other? –Outputs: IMU, engine on-off & gimbals, other? –No general crew interface –Not readily expandable beyond boost functions

MIT Reactions to Proposal General purpose architecture is required –High performance in many applications Reliability can be achieved –Spacecraft environment is low-risk Has to be, with humans present! –MIT’s Polaris experience Interface requires flexibility, expandability –Crew interface, additional subsystems

AGC Architecture (1) Parallel logic –Greater transistor (or gate) count Eased by availability of Fairchild NOR-gate ICs –High performance, especially complex instructions 12 pulse times per memory cycle –Core memory access is parallel anyway –Allows 1’s or 2’s complement notation: MIT took 1’s Logical negation same as numerical negation 1’s complement implies end-around carry –Multiply and divide share normal CPU resources

AGC Architecture (2) Powerful and elegant instruction set –XCH to facilitate time-sharing of erasable (RAM) locations –DV with signed remainder –CCS = (Count, Compare & Skip) for flexible testing & looping –INDEX for indexing and indirect addressing –Support multiple precision (with independent signs per word) –Support general purpose applications, including: Multitasking “executive” to schedule jobs by priority Interpretive code for vector-matrix operations

AGC Architecture (3) Reliable “single string” –No fail-operational (FO) capability for hard failures Restarts handle transient faults gracefully –Takes advantage of low-risk spacecraft environment –Allows use of parallel logic with moderate gate count –Fail-safe (FS) capability provided by AGS –Spacecraft has to have a lot of single-string subsystems anyway IMU and sextant, for instance

AGC Architecture (4) Interfaces –Inputs: flexible set of subsystems and transfer modes PINC/MINC counters and timers UART Channels –Outputs: flexible set of subsystems and transfer modes DINC down-counters UART: SHINC-SHANC Channels –General crew interface (DSKY) Also shared by uplink (the “mechanical boy”)

Summary of the Issue’s Resolution MIT response was nothing if not emphatic –“We are astonished that Bellcomm could have come to this conclusion” – Dick Battin An odd battlefield for so epic a struggle –MSC wasn’t built yet, nor was IAH –MIT, Bellcomm, and NASA met in a motel room across from Hobby Airport People and paper slides perched on 2 king beds

… It Could Have Been Even Better AGC architecture really should have used 2’s complement notation –Easier handling of CDU angles (circle divided into parts) –Could have done logical negation with a central register access Could have had a square root instruction –With a remainder to facilitate multiple precision Wish I’d known then how to build a (CORDIC) simultaneous sine-cosine instruction –Both this and square root are about as complex, and take about the same time, as divide!