PDP-9 control memory

Description of the PDP-9 control memory contents. The control memory is a read-only core memory which consists of 64 process words (i.e. microwords) of 36 bits each.

Wiring matrix

Program listing

Meaning of control bits

Position	Name	Description
00	SUB	Subtract gate
01	EAE
02	EAE-R
03	MBO	Memory buffer output gate
04	MBI	Memory buffer input gate
05	SKPI	Skip input
06	ACO	Accumulator output gate
07	ACI	Accumulator input gate
08	IRI	Instruction regster input gate
09	ARO	Arithmetic register output gate
10	ARI	Arithmetic register input gate
11	CJIT	CAL/JMS/interrupt transfer gate
12	PCO	Program counter output gate
13	PCI	Program counter input gate
14	EAE-P
15	MQO	Multiplier/quotient output gate
16	MQI	Multiplier/quotient input gate
17	DEI	Defer/execute initiate
18	LI	Link input gate
19	AND	AND instruction gate
20	TI	Test for indirect addressing
21	DONE	Instruction done gate
22	AXS	ADD/XOR/SAD instruction gate
23-28	CMA 0-5	Address for next process word, possibly modified
29	EXT	External transfer gate (program break)
30	CONT	Continue to next process word
31	+1	Increment the ADR
32	SM	Start memory cycle
33	ADSO	Address switches output gate
34	KEY	Key gate
35	DCH

CMA modification

The control memory address, CMA, from process word bits 23-28 is heavily modified by random logic before being used for addressing the control memory.

• In address range 00-07 the CMA is OR'ed with a value that depends on the switches:

Value	Switch
1	EXAMINE or DEPOSIT or READ IN
2
3	EXAMINE NEXT or DEPOSIT NEXT or READ IN
5	IO RESET
6	START

• CMA 30 from word 24 is OR'ed with 1 or 2 depending on the result of the TI control bit.

• Data channel requests will similarly "boost" the address.

• In address range 60-77, the lower four bits come from the upper four bits of the instruction register. Hence each process word in this range implements one of the 16 instructions.

Decoded to binary and symbolic form

Address	Process word bits	Symbolic description
00	000000000000000000000000000000000000	CMA/00
01	000010000000000000000000101010001100	MBI ADSO SM CMA/25
02	000000000100000000000000110110001000	ARO SM CMA/33
03	000010000100000000000000101010011000	MBI ARO +1 SM CMA/25
04	000000000000000000000000000000000000	CMA/00
05	000010010010000010100000101110100010	MBI ACI ARI MQI LI KEY CONT CMA/27
06	000010000000010000000000100010001100	MBI PCI ADSO SM CMA/21
07	000010000100000000000000100100110000	MBI ARO +1 CONT CMA/22
10	000000000000100000000000100010001000	PCO SM CMA/21
11	000000001000000000000000110001001000	IRI EXT SM CMA/30
12	000000101010000000000000101000000000	ACO IRI ARI CMA/24
13	000000000010000000000000011100100000	ARI CONT CMA/16
14	000000000100000000000000111110001000	ARO SM CMA/37
15	000000000000000000000000000000000000	CMA/00
16	000000000000000000000000111100001000	SM CMA/36
17	000100000000000000000100010000000000	MBO DONE CMA/10
20	000000000000000000000100010000100000	DONE CONT CMA/10
21	000100000000010000000000010100110000	MBO PCI +1 CONT CMA/12
22	000100010010010010000000001110000100	MBO ACI ARI PCI MQI ADSO CMA/07
23	000100000001000000000001100000110000	MBO CJIT +1 CONT CMA/60
24	000000000000000000001000110000001000	TI SM CMA/30
25	000100000010000000000000101100000010	MBO ARI KEY CMA/26
26	000000000000100000000000100010001000	PCO SM CMA/21
27	000000000000000000000000000000000000	CMA/00
30	000000000001000001000001100000000000	CJIT DEI CMA/60
31	000000000000000001000000101000000000	DEI CMA/24
32	000000000010100000000000100110100000	ARI PCO CONT CMA/23
33	000000001000000000000000101000000000	IRI CMA/24
34	000100000010000000000000010000010001	MBO ARI +1 DCH CMA/10
35	000000000000000000000000000000000000	CMA/00
36	000000000000000000000000011110000001	DCH CMA/17
37	000000000000000000000000010110000001	DCH CMA/13
40	010000000000000000000100010000000000	EAE DONE CMA/10
41	010000100000000010000001011000100000	EAE ACO MQI CONT CMA/54
42	001000100000000010000001011010100000	EAE-R ACO MQI CONT CMA/55
43	010000010000000000000001000010100000	EAE ACI CONT CMA/41
44	000000000000000000000000000000000000	CMA/00
45	000000000000000000000000000000000000	CMA/00
46	000000000000000000000000000000000000	CMA/00
47	000000000000000000000000000000000000	CMA/00
50	000000000010001100000001000100100000	ARI EAE-P MQO CONT CMA/42
51	000010000000100000000001010100001000	MBI PCO SM CMA/52
52	000100000000010000100001010000010000	MBO PCI LI +1 CMA/50
53	001000000010000100000001011100100000	EAE-R ARI MQO CONT CMA/56
54	001000010000000000000001000000100000	EAE-R ACI CONT CMA/40
55	000000010100001000000001010110100000	ACI ARO EAE-P CONT CMA/53
56	000000100000001010000001011110100000	ACO EAE-P MQI CONT CMA/57
57	001000010100000000000001000000100000	EAE-R ACI ARO CONT CMA/40
60	000010000100000000000100010000100000	MBI ARO DONE CONT CMA/10
61	000010100000000000000100010000100000	MBI ACO DONE CONT CMA/10
62	000010000100000000000100010000100000	MBI ARO DONE CONT CMA/10
63	000010000000000000000100010000100000	MBI DONE CONT CMA/10
64	000100010000000000000100010000100000	MBO ACI DONE CONT CMA/10
65	100000010100000000000110010000100000	SUB ACI ARO DONE AXS CONT CMA/10
66	000100010100000000100110010000100000	MBO ACI ARO LI DONE AXS CONT CMA/10
67	000100010100000000100100010000100000	MBO ACI ARO LI DONE CONT CMA/10
70	000000000000000000001000110110001000	TI SM CMA/33
71	000101000010000000000100010000100000	MBO SKPI ARI DONE CONT CMA/10
72	000100010100000000010100010000100000	MBO ACI ARO AND DONE CONT CMA/10
73	100001100010000000000110010000100000	SUB SKPI ACO ARI DONE AXS CONT CMA/10
74	000100000000010000100100010000100000	MBO PCI LI DONE CONT CMA/10
75	010000100010000000100001000110100000	EAE ACO ARI LI CONT CMA/43
76	000000000000000000000000100000000000	CMA/20
77	000001010000000000100100010000100000	SKPI ACI LI DONE CONT CMA/10

Explanation

When the machine is powered on, control memory loops in a "nop" word at CMA 00. When the START switch is pressed, the CMA is "boosted" to 06 by random logic decoding the state of the switches.

• 06: ADSO MBI PCI SM CMA/21
  ADSO gates output from the address switches; MBI and PCI gates input to the MB and PC registers. CM starts memory acess. Processing enters in the middle of the fetch phase at CMA 21.

The normal start of the instruction flow is at CMA 10. All instructions end by looping back there.

• 10: PCO SM CMA/21
  PCO gates output from the PC register. Random logic sets MBI which gates input to the MB register, thus setting it to PC. SM starts memory access.
• A new memory cycle starts, reading MB into MA.
• 21: MBO +1 PCI CONT CMA/12
  MBO puts MB into the adder, and +1 increments the address by one. The sum is put into PC.
• The read portion of the memory cycle is now ready. Random logic sets SAO and MBI, which transfers data from the memory SA "sense amplifiers" to MB.
• 12: IRI ACO ARI CMA/24
  IRI sets the IR register from SA. ACO and ARI copies the accumulator contents to the AR register.
• 24: TI SM CMA/30
  TI makes a check for indirect addressing. SM starts a new memory access to retrieve the instruction operand.
• A new memory cycle starts.
• At this point, CMA 30 may be "boosted" by random logic. If there's indirect addressing, processing continues at CMA 31. If the IR holds a CAL, DAC, JMS, or DZM instruction, processing continues at 32. Otherwise, proceed to the unboosted address 30.
  • 30: DEI CJIT CMA/60
    DEI ensures MBI and SAO are set to retrieve the memory operand. CJIT activates a check for ISZ with operand 0, and increments the PC if so. The next CMA is "boosted" by putting the instruction opcode in the lower four bits, thus dispatching to CMA 60-77.
  • 31: DEI CMA/24
    DEI clears the indirect addressing bit, and processing loops back to CMA 24 to start a new memory cycle using MB as the address.
  • 32: ARI PCO CONT CMA/23
    The PC is saved in the AR register for the benefit of CAL and JMS.
    23: MBO CJIT +1 CONT CMA/60
    MBO sends the MB through the adder, and +1 increments it. CJIT checks for CAL or JMS to set PCI and store the incremented result the PC. Dispatch to CMA 60-77.
• The instuction execute phase dispatches to the CMA region 60-77. Three instructions, XCT, EAE, and IOT, have a more complicated flow.
  • 60-77: [various] DONE CONT CMA/10
    Most instructions do their thing and loop back to CMA 10.
  • XCT 70: TI SM CMA/33
    Start a new memory access to fetch the instruction.
    33: IRI CMA/24
    Put the opcode in IR and loop back to CMA 24.
  • EAE 75: EAE ACO ARI LI CONT CMA/43
    Extended arithmetic is implemented in CMA 40-57 and will eventually reach a process word with DONE CONT CMA/10.
  • IOT 76: CMA/20
    Random logic does all the work for the I/O transfer instructions.
    20: DONE CONT CMA/10
    Finish I/O and loop back to CMA 10.