μCOM-4 ISA - NEC
< nec‎ | μcom-4

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.)

## 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

## References

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