The μCOM-4 ISA was an instruction set architecture composed of 80 instructions that were implemented by the NEC in their μCOM-4 family of microcontrollers. The μCOM-43 family implemented the complete ISA, while the μCOM-44 and μCOM-45 families implemented a strict subset of it. (Note that the μCOM-42 family had its own instruction set.)

Block diagram

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Registers

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Features

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Instructions

The μCOM-43 has support for all 80 instructions. The μCOM-44/μCOM-45 support a strict subset of it composed of 58 instruction. The majority of those 22 instructions deal with the added features only found in the μCOM-43 models.

Mnemonic	OPCode	Bytes	Cycles	Function	Meaning	Skip Condition
CLA	90	1	1	ACC ← 0	Clear accumulator
CMA	10	1	1	ACC ← ~ACC	1-s complement accumulator
CIA	11	1	1	ACC ← ~ACC + 1	2-s complement accumulator
INC	0D	1	1 + skip	ACC ← ACC + 1	Increment accumulator, skip if overflow	on overflow
DEC	0F	1	1 + skip	ACC ← ACC - 1	Decrement accumulator, skip if underflow	on underflow
CLC	0B	1	1	C ← 0	Clear carry flag
STC	1B	1	1	C ← 1	Set carry flag
XC	1A	1	1	C ↔ C'	Exchange carry flag with carry save
RAR	30	1	1	C → ACC → C	Rotate accumulator right through carry flag
INM	1D	1	1 + skip	(DP) = (DP) + 1	Increment RAM content pointed by DP, skip if result is zero	on (DP) = 0
DEM	1F	1	1 + skip	(DP) = (DP) - 1	Decrement RAM content pointed by DP, skip if result is 0Fh	on (DP) = 0Fh
AD	08	1	1 + skip	ACC ← ACC + (DP)	Add RAM content pointed by DP to accumulator, skip if overflow	on overflow
ADS	09	1	1 + skip	ACC ← ACC + (DP) + C C ← 1 on overflow	Add RAM content pointed by DP with carry to accumulator If overflow set carry flag and skip	on overflow
ADC	19	1	1	ACC ← ACC + (DP) + C C ← 1 on overflow	Add RAM content pointed by DP with carry to accumulator If overflow set carry flag
DAA	06	1	1	ACC ← ACC + 6	Decimal adjust accumulator for BCD addition
DAS	0A	1	1	ACC ← ACC + 10	Decimal adjust accumulator for BCD subtraction
EXL	18	1	1	ACC ← ACC ^ (DP)	Exclusive-or accumulator with RAM content pointed by DP
LI	90 + D[3:0]	1	1	ACC ← D[3:0]	Load accumulator with immediate data D[3:0]
S	04	1	1	(DP) ← ACC	Store accumulator into RAM location pointed by DP
L	38	1	1	ACC ← (DP)	Load accumulator with RAM content pointed by DP
LM	38 + D[1:0]	1	1	ACC ← (DP) DPH ← DPH ^ D[1:0]	Load accumulator with RAM content pointed by DP; Exclusive-or DPH with immediate data D[1:0]
X	28	1	1	ACC ↔ (DP)	Exchange accumulator with RAM content pointed by DP
XM	28 + D[1:0]	1	1	ACC ↔ (DP) DPH ← DPH ^ D[1:0]	Exchange accumulator with RAM content pointed by DP; Exclusive-or DPH with immediate data D[1:0]
XD	2C	1	1 + skip
XMD	2C + D[1:0]	1	1 + skip
XI	3C	1	1 + skip
XMI	3C + D[1:0]	1	1 + skip
LDI	15 00 + D[6:0]	2	2
LDZ	80 + D[3:0]	1	1
DED	13	1	1 + skip
IND	33	1	1 + skip
TAL	07	1	1
TLA	12	1	1
XHX	4F	1	2
XLY	4E	1	2
THX	47	1	2
TLY	46	1	2
XAZ	4A	1	2
XAW	4B	1	2
TAZ	42	1	2
TAW	43	1	2
XHR	4D	1	2
XLS	4C	1	2
SMB	78 + D[1:0]	1	1
RMB	68 + D[1:0]	1	1
TMB	58 + D[1:0]	1	1 + skip
TAB	24 + D[1:0]	1	1 + skip
CMB	34 + D[1:0]	1	1 + skip
SFB	7C + D[1:0]	1	2
RFB	6C + D[1:0]	1	2
FBT	5C + D[1:0]	1	2 + skip
FBF	20 + D[1:0]	1	2 + skip
CM	0C	1	1 + skip
CI	17 C0 + D[3:0]	2	2 + skip
CLI	16 E0 + D[3:0]	2	2 + skip
TC	04	1	1 + skip
TIT	03	1	1 + skip
JCP	C0 + D[5:0]	1	1
JMP	A0 + D[10:8] 00 + D[7:0]	2	2
JPA	41	1	2
EI	31	1	1
DI	01	1	1
CZP	B0 + D[3:0]	1	1
CAL	A8 + D[10:8] 00 + D[7:0]	2	2
RT	48	1	2
RTS	49	1	2 + skip
STM	14 80 + D[5:0]	2	2
TTM	05	1	1 + skip
SEB	74 + D[1:0]	1	2
REB	64 + D[1:0]	1	2
SPB	70 + D[1:0]	1	1
RPB	60 + D[1:0]	1	1
TPA	54 + D[1:0]	1	2 + skip
TPB	50 + D[1:0]	1	1 + skip
OE	44	1	2
OP	0E	1	1
OCD	1E 00 + D[7:0]	2	2	PORT[C,D] ← I7-I0
IA	40	1	2	ACC ← PORT[A]
IP	32	1	1	ACC ← PORT[DPL]
NOP	00	1	1	no operation

  - μCOM-43 only

Documents

• ISA data sheet

References

• NEC Electronics (Europe) GmbH, 1982 Catalog, "μCOM-4: 4-Bit Single Chip Microcomputer Family", 143-176 pp.

See also

• μCOM-4
• μCOM-8
• μCOM-8 ISA