This is one of the densest parts of this section. The book first shows a machine-instruction format, then a 20-bit microinstruction format that controls the datapath.

Machine-instruction format shown on p. 337

The figure labels bit 15 as I, bits 14–12 as the opcode, and bits 11–0 as the address. The nearby printed prose appears inconsistent with these bit boundaries, so for understanding the diagram itself is the safer reference.

IOpcodeAddress

20-bit control-memory microinstruction

Field	Bits
F1	3
F2	3
F3	3
CD	2
BR	2
AD	7

F1, F2, F3: microoperation fields. One microoperation may be chosen from each field.

CD: condition-selection field.

BR: branch-operation field.

AD: branch address.

Each 3-bit F field can encode 8 values. The book uses 000 = NOP, leaving seven active microoperations. At most three compatible microoperations can be selected simultaneously—one from each field.

F1 microoperations

Code	Microoperation	Symbol
000	None	NOP
001	AC ← AC + DR	ADD
010	AC ← 0	CLRAC
011	AC ← AC + 1	INCAC
100	AC ← DR	DRTAC
101	AR ← DR(0–10)	DRTAR
110	AR ← PC	PCTAR
111	M[AR] ← DR	WRITE

F2 microoperations

Code	Microoperation	Symbol
000	None	NOP
001	AC ← AC − DR	SUB
010	AC ← AC ∨ DR	OR
011	AC ← AC ∧ DR	AND
100	DR ← M[AR]	READ
101	DR ← AC	ACTDR
110	DR ← DR + 1	INCDR
111	DR(0–10) ← PC	PCTDR

F3 microoperations

Code	Microoperation	Symbol
000	None	NOP
001	AC ← AC ⊕ DR	XOR
010	AC ← complement of AC	COM
011	AC ← shl AC	SHL
100	AC ← shr AC	SHR
101	PC ← PC + 1	INCPC
110	PC ← AR	ARTPC
111	Reserved	—

Condition (CD) field

CD	Condition	Symbol
00	Always = 1	U
01	DR(15)	I
10	AC(15)	S
11	AC = 0	Z

Branch (BR) field

BR	Symbol	Meaning
00	JMP	If condition=1, CAR←AD; otherwise CAR←CAR+1.
01	CALL	If condition=1, CAR←AD and SBR←CAR+1; otherwise CAR←CAR+1.
10	RET	CAR←SBR.
11	MAP	Forms a control address from instruction bits.

Exam Focus

20-bit format = 3+3+3+2+2+7.
F1/F2/F3 choose datapath microoperations.
CD chooses a condition; BR chooses the sequencing action; AD supplies a branch address.
Conflicting microoperations must not be selected simultaneously.