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Apollo Guidance Computer and DSKY
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| Invented by | MIT Instrumentation Laboratory |
|---|---|
| Manufacturer | Raytheon |
| Introduced | August 1966 |
| Discontinued | July 1975 |
| Type | Avionics Guidance Computer |
| Processor | Discrete IC RTL based |
| Frequency | 2.048 MHz |
| Memory | 16-bit wordlength, 2048 words RAM (magnetic core memory), 36,864 words ROM (core rope memory) |
| Ports | DSKY, IMU, Hand Controller, Rendezvous Radar (CM), Landing Radar (LM), Telemetry Receiver, Engine Command, Reaction Control System |
| Power consumption | 55W[1]:120 |
| Weight | 70 lb (32 kg) |
| Dimensions | 24x12.5x6.5 inches (61x32x17 cm) |
Astronauts communicated with the AGC using a numeric display and keypad called the DSKY. The AGC and its DSKY user interface were developed in the early 1960s for the Apollo program by the MIT Instrumentation Laboratory. The AGC is notable for being one of the first integrated circuit-based computers.
Contents
Operation
Each flight to the Moon (with the exception of Apollo 8, which did not take a Lunar Module on its lunar orbit mission) had two AGCs, one each in the Command Module and the Lunar Module. The AGC in the Command Module was at the center of that spacecraft's guidance, navigation and control (GNC) system. The AGC in the Lunar Module ran its Apollo PGNCS (Primary Guidance, Navigation and Control System), with the acronym pronounced as pings.
- The Launch Vehicle Digital Computer (LVDC) on the Saturn V booster instrumentation ring
- the Abort Guidance System (AGS) of the Lunar Module, to be used in the event of failure of the LM PGNCS. The AGS could be used to take off from the Moon, and to rendezvous with the Command Module, but not to land
Design
The AGC was designed at the MIT Instrumentation Laboratory under Charles Stark Draper, with hardware design led by Eldon C. Hall.[1] Early architectural work came from J.H. Laning Jr., Albert Hopkins, Ramon Alonso,[3] [4] and Hugh Blair-Smith.[5] The flight hardware was fabricated by Raytheon, whose Herb Thaler[6] was also on the architectural team.The Apollo flight computer was the first to use integrated circuits (ICs). While the Block I version used 4,100 ICs, each containing a single 3-input NOR gate, the later Block II version (used in the crewed flights) used 2,800 ICs, each with dual 3-input NOR gates.[1]:34 The ICs, from Fairchild Semiconductor, were implemented using resistor-transistor logic (RTL) in a flat-pack. They were connected via wire wrap, and the wiring was then embedded in cast epoxy plastic. The use of a single type of IC (the dual NOR3) throughout the AGC avoided problems that plagued another early IC computer design, the Minuteman II guidance computer, which used a mix of diode-transistor logic and diode logic gates.
The computer had 2048 words of erasable magnetic core memory and 36 kilowords of read-only core rope memory. Both had cycle times of 11.72 micro-seconds. The memory word length was 16 bits: 15 bits of data and 1 odd-parity bit. The CPU-internal 16-bit word format was 14 bits of data, 1 overflow bit, and 1 sign bit (ones' complement representation).
DSKY interface
The numerals were displayed via green high-voltage electroluminescent seven-segment displays. The segments were driven by electromechanical relays, which limited the display update rate (Block II used faster silicon controlled rectifiers). Three 5-digit signed numbers could also be displayed in octal or decimal, and were typically used to display vectors such as space craft attitude or a required velocity change (delta-V). Although data were stored internally in metric units, they were displayed as United States customary units. This calculator-style interface[nb 1] was the first of its kind, the prototype for all similar digital control panel interfaces.
The Command Module had two DSKYs connected to its AGC: one located on the main instrument panel and a second located in the lower equipment bay near a sextant used for aligning the inertial guidance platform. The Lunar Module had a single DSKY for its AGC. A flight director attitude indicator (FDAI), controlled by the AGC, was located above the DSKY on the commander's console and on the LM.
In 2009, a DSKY was sold in a public auction held by Heritage Auctions for $50,788.[7]
Timing
The AGC timing reference came from a 2.048 MHz crystal clock. The clock was divided by two to produce a four-phase 1.024 MHz clock which the AGC used to perform internal operations. The 1.024 MHz clock was also divided by two to produce a 512 kHz signal called the master frequency; this signal was used to synchronize external Apollo spacecraft systems.The master frequency was further divided through a scaler, first by five using a ring counter to produce a 102.4 kHz signal. This was then divided by two through 17 successive stages called F1 (51.2 kHz) through F17 (0.78125 Hz). The F10 stage (100 Hz) was fed back into the AGC to increment the real-time clock and other involuntary counters using Pinc (discussed below). The F17 stage was used to intermittently run the AGC when it was operating in the standby mode.
Central registers
The AGC had four 16-bit registers for general computational use, called the central registers:| A : | The accumulator, for general computation |
| Z : | The program counter - the address of the next instruction to be executed |
| Q : | The remainder from the DV instruction, and the return address after TC instructions |
| LP : | The lower product after MP instructions |
Other registers
| S | : | 12-bit memory address register, the lower portion of the memory address |
| Bank/Fbank | : | 4-bit ROM bank register, to select the 1 kiloword ROM bank when addressing in the fixed-switchable mode |
| Ebank | : | 3-bit RAM bank register, to select the 256-word RAM bank when addressing in the erasable-switchable mode |
| Sbank (super-bank) | : | 1-bit extension to Fbank, required because the last 4 kilowords of the 36-kiloword ROM was not reachable using Fbank alone |
| SQ | : | 4-bit sequence register; the current instruction |
| G | : | 16-bit memory buffer register, to hold data words moving to and from memory |
| X | : | The 'x' input to the adder (the adder was used to perform all 1's complement arithmetic) or the increment to the program counter (Z register) |
| Y | : | The other ('y') input to the adder |
| U | : | Not really a register, but the output of the adder (the 1's complement sum of the contents of registers X and Y) |
| B | : | General-purpose buffer register, also used to pre-fetch the next instruction. At the start of the next instruction, the upper bits of B (containing the next op. code) were copied to SQ, and the lower bits (the address) were copied to S. |
| C | : | Not a separate register, but the 1's complement of the B register |
| IN | : | Four 16-bit input registers |
| OUT | : | Five 16-bit output registers |
Instruction set
The instruction format used 3 bits for opcode, and 12 bits for address. Block I had 11 instructions:TC, CCS, INDEX, XCH, CS, TS, AD, and MASK (basic), and SU, MP, and DV (extra). The first eight, called basic instructions, were directly accessed by the 3-bit op. code. The final three were denoted as extracode instructions because they were accessed by performing a special type of INDEX instruction (called EXTEND) immediately before the instruction.The Block I AGC instructions consisted of the following:
TC(transfer control)- An unconditional branch to the address specified by the instruction.
The return address was automatically stored in the Q register, so the
TCinstruction could be used for subroutine calls. CCS(count, compare, and skip)- A complex conditional branch instruction. The A register was loaded
with data retrieved from the address specified by the instruction.
(Because the AGC uses ones' complement notation, there are two representations of zero. When all bits are set to zero, this is called plus zero. If all bits are set to one, this is called minus zero.) The diminished absolute value
(DABS) of the data was then computed and stored in the A register. If
the number was greater than zero, the DABS decrements the value by 1; if
the number was negative, it is complemented before the decrement is
applied—this is the absolute value. Diminished means "decremented
but not below zero". Therefore, when the AGC performs the DABS
function, positive numbers will head toward plus zero, and so will
negative numbers but first revealing their negativity via the four-way
skip below. The final step in
CCSis a four-way skip, depending upon the data in register A before the DABS. If register A was greater than 0,CCSskips to the first instruction immediately afterCCS. If register A contained plus zero,CCSskips to the second instruction afterCCS. Less than zero causes a skip to the third instruction afterCCS, and minus zero skips to the fourth instruction afterCCS. The primary purpose of the count was to allow an ordinary loop, controlled by a positive counter, to end in aCCSand aTCto the beginning of the loop, equivalent to an IBM 360'sBCT. The absolute value function was deemed important enough to be built into this instruction; when used for only this purpose, the sequence after theCCSwasTC*+2,TC*+2,ADONE. A curious side effect was the creation and use ofCCS-holes when the value being tested was known to be never positive, which occurred more often than one might suppose. That left two whole words unoccupied, and a special committee was responsible for assigning data constants to these holes. INDEX- Add the data retrieved at the address specified by the instruction to the next instruction.
INDEXcan be used to add or subtract an index value to the base address specified by the operand of the instruction that followsINDEX. This method is used to implement arrays and table look-ups; since the addition was done on both whole words, it was also used to modify the op. code in a following (extracode) instruction, and on rare occasions both functions at once. RESUME- A special instance of
INDEX(INDEX25). This is the instruction used to return from interrupts. It causes execution to resume at the interrupted location. XCH(exchange)- Exchange the contents of memory with the contents of the A register.
If the specified memory address is in fixed (read-only) memory, the
memory contents are not affected, and this instruction simply loads
register A. If it is in erasable memory, overflow "correction" is
achieved by storing the leftmost of the 16 bits in A as the sign bit in
memory, but there is no exceptional behavior like that of
TS. CS(clear and subtract)- Load register A with the one's complement of the data referenced by the specified memory address.
TS(transfer to storage)- Store register A at the specified memory address.
TSalso detects, and corrects for, overflows in such a way as to propagate a carry for multi-precision add/subtract. If the result has no overflow (leftmost 2 bits of A the same), nothing special happens; if there is overflow (those 2 bits differ), the leftmost one goes the memory as the sign bit, register A is changed to +1 or -1 accordingly, and control skips to the second instruction following theTS. Whenever overflow is a possible but abnormal event, theTSwas followed by aTCto the no-overflow logic; when it is a normal possibility (as in multi-precision add/subtract), theTSis followed byCAFZERO (CAF=XCHto fixed memory) to complete the formation of the carry (+1, 0, or -1) into the next higher-precision word. Angles were kept in single precision, distances and velocities in double precision, and elapsed time in triple precision. AD(add)- Add the contents of memory to register A and store the result in A.
The 2 leftmost bits of A may be different (overflow state) before and/or
after the
AD. The fact that overflow is a state rather than an event forgives limited extents of overflow when adding more than two numbers, as long as none of the intermediate totals exceed twice the capacity of a word. MASK- Perform a bit-wise (boolean) and of memory with register A and store the result in register A.
MP(multiply)- Multiply the contents of register A by the data at the referenced memory address and store the high-order product in register A and the low-order product in register LP. The parts of the product agree in sign.
DV(divide)- Divide the contents of register A by the data at the referenced memory address. Store the quotient in register A and the absolute value of the remainder in register Q. Unlike modern machines, fixed-point numbers were treated as fractions (notional decimal point just to right of the sign bit), so you could produce garbage if the divisor was not larger than the dividend; there was no protection against that situation. In the Block II AGC, a double-precision dividend started in A and L (the Block II LP), and the correctly signed remainder was delivered in L. That considerably simplified the subroutine for double precision division.
SU(subtract)- Subtract (one's complement) the data at the referenced memory address from the contents of register A and store the result in A.
MP) used 8 subsequences: an initial one called MP0, followed by an MP1 subsequence which was repeated 6 times, and then terminated by an MP3 subsequence. This was reduced to 3 subsequences in Block II.Each timing pulse in a subsequence could trigger up to 5 control pulses. The control pulses were the signals which did the actual work of the instruction, such as reading the contents of a register onto the bus, or writing data from the bus into a register.
Memory
The Block I AGC initially had 12 kilowords of fixed memory, but this was later increased to 24 kilowords. Block II had 32 kilowords of fixed memory and 4 kilowords of erasable memory.
The AGC transferred data to and from memory through the G register in a process called the memory cycle. The memory cycle took 12 timing pulses (11.72 μs). The cycle began at timing pulse 1 (TP1) when the AGC loaded the memory address to be fetched into the S register. The memory hardware retrieved the data word from memory at the address specified by the S register. Words from erasable memory were deposited into the G register by timing pulse 6 (TP6); words from fixed memory were available by timing pulse 7. The retrieved memory word was then available in the G register for AGC access during timing pulses 7 through 10. After timing pulse 10, the data in the G register was written back to memory.
The AGC memory cycle occurred continuously during AGC operation. Instructions needing memory data had to access it during timing pulses 7-10. If the AGC changed the memory word in the G register, the changed word was written back to memory after timing pulse 10. In this way, data words cycled continuously from memory to the G register and then back again to memory.
The lower 15 bits of each memory word held AGC instructions or data, with each word being protected by a 16th odd parity bit. This bit was set to 1 or 0 by a parity generator circuit so a count of the 1s in each memory word would always produce an odd number. A parity checking circuit tested the parity bit during each memory cycle; if the bit didn't match the expected value, the memory word was assumed to be corrupted and a parity alarm panel light was illuminated.
Interrupts and involuntary counters
The AGC had five vectored interrupts:- Dsrupt was triggered at regular intervals to update the user display (DSKY).
- Erupt was generated by various hardware failures or alarms.
- Keyrupt signaled a key press from the user's keyboard.
- T3Rrupt was generated at regular intervals from a hardware timer to update the AGC's real-time clock.
- Uprupt was generated each time a 16-bit word of uplink data was loaded into the AGC.
The AGC also had 20 involuntary counters. These were memory locations which functioned as up/down counters, or shift registers. The counters would increment, decrement, or shift in response to internal inputs. The increment (Pinc), decrement (Minc), or shift (Shinc) was handled by one subsequence of microinstructions inserted between any two regular instructions.
Interrupts could be triggered when the counters overflowed. The T3rupt and Dsrupt interrupts were produced when their counters, driven by a 100 Hz hardware clock, overflowed after executing many Pinc subsequences. The Uprupt interrupt was triggered after its counter, executing the Shinc subsequence, had shifted 16 bits of uplink data into the AGC.
Standby mode
The AGC had a power-saving mode controlled by a standby allowed switch. This mode turned off the AGC power, except for the 2.048 MHz clock and the scaler. The F17 signal from the scaler turned the AGC power and the AGC back on at 1.28 second intervals. In this mode, the AGC performed essential functions, checked the standby allowed switch, and, if still enabled, turned off the power and went back to sleep until the next F17 signal.In the standby mode, the AGC slept most of the time; therefore it was not awake to perform the Pinc instruction needed to update the AGC's real time clock at 10 ms intervals. To compensate, one of the functions performed by the AGC each time it awoke in the standby mode was to update the real time clock by 1.28 seconds.
The standby mode was designed to reduce power by 5 to 10 W (from 70 W) during midcourse flight when the AGC was not needed. However, in practice, the AGC was left on during all phases of the mission and this feature was never used.
Data buses
The AGC had a 16-bit read bus and a 16-bit write bus. Data from central registers (A, Q, Z, or LP), or other internal registers could be gated onto the read bus with a control signal. The read bus connected to the write bus through a non-inverting buffer, so any data appearing on the read bus also appeared on the write bus. Other control signals could copy write bus data back into the registers.Data transfers worked like this: To move the address of the next instruction from the B register to the S register, an RB (read B) control signal was issued; this caused the address to move from register B to the read bus, and then to the write bus. A WS (write S) control signal moved the address from the write bus into the S register.
Several registers could be read onto the read bus simultaneously. When this occurred, data from each register was inclusive-ORed onto the bus. This inclusive-OR feature was used to implement the Mask instruction, which was a logical AND operation. Because the AGC had no native ability to do a logical AND, but could do a logical OR through the bus and could complement (invert) data through the C register, De Morgan's theorem was used to implement the equivalent of a logical AND. This was accomplished by inverting both operands, performing a logical OR through the bus, and then inverting the result.
Software
When the design requirements for the AGC were defined, necessary software and programming techniques didn't exist so it had to be designed from scratch.AGC software was written in AGC assembly language and stored on rope memory. There was a simple real-time operating system consisting of the Exec, a batch job-scheduling system that could run up to eight 'jobs' at a time using cooperative multi-tasking (each job had to periodically surrender control back to the Exec which then checked if there was any waiting job with higher priority). There was also an interrupt-driven component called the Waitlist which could schedule multiple timer-driven 'tasks'. The tasks were short threads of execution which could reschedule themselves for re-execution on the Waitlist, or could kick off a longer operation by starting a 'job' with the Exec.
The Exec jobs were priority-based. The lowest priority job, called the dummy job, was always present. It did diagnostic checks and controlled a green computer activity light on the DSKY: If the dummy job was running, this meant the computer had nothing better to do, so the light was turned off. The dummy job exited if there was some higher priority job to be done and this was indicated by the computer activity light being illuminated.
The AGC also had a sophisticated software interpreter, developed by MIT, that implemented a virtual machine with more complex and capable pseudo-instructions than the native AGC. These instructions simplified the navigational programs. Interpreted code, which featured double precision trigonometric, scalar and vector arithmetic (16 and 24-bit), even an
MXV
(matrix × vector) instruction, could be mixed with native AGC code.
While the execution time of the pseudo-instructions was increased (due
to the need to interpret these instructions at runtime) the interpreter
provided many more instructions than AGC natively supported and the
memory requirements were much lower than in the case of adding these
instructions to the AGC native language which would require additional
memory built into the computer (at that time the memory capacity was
very expensive). The average pseudo-instruction required about 24 ms to
execute. The assembler and version control system, named YUL for an early prototype Christmas Computer,[8] enforced proper transitions between native and interpreted code.A set of interrupt-driven user interface routines called Pinball provided keyboard and display services for the jobs and tasks running on the AGC. A rich set of user-accessible routines were provided to let the operator (astronaut) display the contents of various memory locations in octal or decimal in groups of 1, 2, or 3 registers at a time. Monitor routines were provided so the operator could initiate a task to periodically redisplay the contents of certain memory locations. Jobs could be initiated. The Pinball routines performed the (very rough) equivalent of the UNIX shell.
The bulk of the software was on read-only rope memory and thus couldn't be changed in operation, but some key parts of the software were stored in standard read-write magnetic-core memory and could be overwritten by the astronauts using the DSKY interface, as was done on Apollo 14.
Block II
A Block II version of the AGC was designed in 1966. It retained the basic Block I architecture, but increased erasable memory from 1 to 2 kilowords. Fixed memory was expanded from 24 to 36 kilowords. Instructions were expanded from 11 to 34 and I/O channels were implemented to replace the I/O registers on Block I. The Block II version is the one that actually flew to the moon. Block I was used during the unmanned Apollo 4 and 6 flights, and was on board the ill-fated Apollo I.The decision to expand the memory and instruction set for Block II, but to retain the Block I's restrictive 3-bit op. code and 12-bit address had interesting design consequences. Various tricks were employed to squeeze in additional instructions, such as having special memory addresses which, when referenced, would implement a certain function. For instance, an
INDEX to address 25 triggered the RESUME instruction to return from an interrupt. Likewise, INDEX 17 performed an INHINT instruction (inhibit interrupts), while INDEX 16 reenabled them (RELINT). Other instructions were implemented by preceding them with a special version of INDEX called EXTEND
which arithmetically modified the 3-bit op. code by employing the
overflow bit to extend it. The address spaces were extended by employing
the Bank (fixed) and Ebank (erasable) registers, so the only memory of
either type that could be addressed at any given time was the current
bank, plus the small amount of fixed-fixed memory and the erasable
memory. In addition, the bank register could address a maximum of 32
kilowords, so an Sbank (super-bank) register was required to access the
last 4 kilowords. All across-bank subroutine calls had to be initiated
from fixed-fixed memory through special functions to restore the
original bank during the return: essentially a system of far pointers.The Block II AGC also has the mysterious and poorly documented
EDRUPT instruction (the name may be a contraction of Ed's Interrupt, after Ed Smally, the programmer who requested it) which is used a total of once in the Apollo software: in the Digital Autopilot of the Lunar Module.
At this time, while the general operation of the instruction is
understood, the precise details are still hazy, and it is believed to be
responsible for problems emulating the LEM AGC Luminary software.[citation needed]PGNCS trouble
PGNCS generated unanticipated warnings during Apollo 11's lunar descent, with the AGC showing a 1201 alarm ("Executive overflow - no vacant areas") and a 1202 alarm ("Executive overflow - no core sets").[9] The cause was a rapid, steady stream of spurious cycle steals from the rendezvous radar, intentionally left on standby during the descent in case it was needed for an abort.[10][11]During this part of the approach the processor would normally be almost 85% loaded. The extra 6400 cycle steals per second added the equivalent of 13% load, leaving just enough time for all scheduled tasks to run to completion. Five minutes into the descent Buzz Aldrin gave the computer the command 1668 which instructed it to calculate and display DELTAH (the difference between altitude sensed by the radar and the computed altitude). This added an additional 10% to the processor workload causing executive overflow and a 1202 alarm. After being given the "GO" from Houston Aldrin entered 1668 again and another 1202 alarm occurred. When reporting the second alarm Aldrin added the comment "It appears to come up when we have a 1668 up". Happily for Apollo 11, the AGC software had been designed with priority scheduling. Just as it had been designed to do, the software automatically recovered, deleting lower priority tasks including the 1668 display task, to complete its critical guidance and control tasks. Guidance controller Steve Bales and his support team that included Jack Garman issued several "GO" calls and the landing was successful. For his role, Bales received the US Presidential Medal of Freedom on behalf of the entire control center team and the three Apollo astronauts.[12]
The problem was not a programming error in the AGC, nor was it pilot error. It was a peripheral hardware design bug that was already known and documented by Apollo 5 engineers.[13] However because the problem had only occurred once during testing they concluded that it was safer to fly with the existing hardware that they had already tested, than to fly with a newer but largely untested radar system. In the actual hardware, the position of the rendezvous radar was encoded with synchros excited by a different source of 800 Hz AC than the one used by the computer as a timing reference. The two 800 Hz sources were frequency locked but not phase locked, and the small random phase variations made it appear as though the antenna was rapidly "dithering" in position even though it was completely stationary. These phantom movements generated the rapid series of cycle steals.
00404 error code
The computer's other error codes included error 00404, which was shorthand for "IMU orientation unknown". Since the Inertial Measurement Unit device literally told the craft where to go, this has been compared to the HTTP 404 not found or browser navigation error code used on the World Wide Web. However, the later familiar HTTP error code did not originate with the AGC.[14]Applications outside Apollo
The AGC was also used for the U.S. Navy's Deep Submergence Rescue Vehicle[16]
See also
- Apollo PGNCS - the Apollo Primary Guidance and Navigation System
- AP-101 (IBM S/360-derived) computers used in the Space Shuttle
- History of computer hardware
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