8008 ISA
Developer	Intel Datapoint Corporation
Implementation	8008
Dev model	proprietary
Design	Von Neumann architecture
Data word size	8 bit 1 octets 2 nibbles
Instruction word size	8 bit 1 octets
Instructions	48
Introduction	1972
Version	1
Format	register-register
Endianness	bi-endian
Registers	0
GPRs	7 (scratchpad)
ISAs • By Company • By Inst • By Data

The 8008 ISA (or MCS-8 ISA) was an instruction set architecture introduced by Intel in 1972 and was used in the 8008 and 8008-1 microprocessors.

This ISA has an 8-bit data and address bus. This architecture included seven 8-bit registers, 48 instructions, and interrupt capability.

Registers

The 8008 had seven scratchpad registers. A few of them had additional capabilities - A is used an an accumulator register. Registers H & L are high-order and low-order words of a 14-bit address.

Register	Size	Purpose
A	8 bit	Accumulator
B	8 bit	GP
C	8 bit	GP
D	8 bit	GP
E	8 bit	GP
H	8 bit	High-order word
L	8 bit	Low-order word

ISA

Data on the 8008 is always stored in an 8-bit binary integer.

Data Word

D₇ D₆ D₅ D₄ D₃ D₂ D₁ D₀

Instructions can be made of 1-3 bytes depending on operation. Multi-byte instructions must be stored in successive order in memory. Typical operations involving register-register operations such as arithmetic and logic operations only require one byte and take the following form:

1-Byte Inst

D₇ D₆ D₅ D₄ D₃ D₂ D₁ D₀

OPCode

Instructions that involve an immediate value have 2 bytes. The first bite stores the opcode and the second byte stores the 8-bit value.

2-Byte Inst

D₇ D₆ D₅ D₄ D₃ D₂ D₁ D₀

D₇ D₆ D₅ D₄ D₃ D₂ D₁ D₀

OPCode

Imm Value

Instructions involving an address (such as CALL and JUMP) require a 14-bit address. This is done via a 3-byte instruction where the first byte is the opcode, the second byte is the low-order word, and the third byte is the high-order word. Note that the 2 MSBs on the high-order word are don't cares.

3-Byte Inst

D₇ D₆ D₅ D₄ D₃ D₂ D₁ D₀

X₇ X₆ D₅ D₄ D₃ D₂ D₁ D₀

D₇ D₆ D₅ D₄ D₃ D₂ D₁ D₀

OPCode

High-Order Addrs

Low-Order Addrs

Listing

The 8008 ISA has 48 instructions broken a number of groups:

• Index Register Instructions
• Accumulator Group Instructions
• Program Counter and Stack Control Instructions
• Input/Output Instructions
• Machine Instructions

In 1972, Intel introduce their first set of mnemonics for their instructions. This original set consists of three characters which meant it could easily be encoded into a lookup table. A few years later, when they released their 8080 and its instruction set, they also revamped the 8008 mnemonics to match the 8080's ones more closely. The newer mnemonics resemble a primitive version of modern x86 mnemonics. Since 8008 programs can be found using bother mnemonics, both the "old" and the "new" mnemonics are listed below. Both mnemonics results in identical binary values.

8008 ISA
Mnemonic (old)
Index Register Instructions Load instructions do not affect any flag. The Inc and Dec instructions affect all flags except carry.
LRdRs	MOV Rd, Rs	11 DDD SSS2	Rd = Rs
LRdM	MOV Rd, M	11 DDD 1112	Rd = Mem
LMRs	MOV Rs, M	11 111 SSS2	Mem = Rs
LRdI	MVI Rd, Imm	00 DDD 1102 BB BBB BBB2	Rd = B7...B0
LMI	MVI M, Imm	00 111 1102 BB BBB BBB2	Mem = B7...B0
INRd	INR Rd	00 DDD 0002	Rd = Rd + 1 (Rd ≠ A)
DCRd	DCR Rd	00 DDD 0012	Rd = Rd - 1 (Rd ≠ A)
Accumulator Group Instructions The result of an ALU instruction affect all flags. The rotation instructions only affect the carry flag.
Program Counter and Stack Control Instructions
Input/Output Instructions
INP	IN	01 00M MM12	A = PORT[MMM]
OUT	OUT	01 RRM MM12	PORT[RRMMM] = A (RR ≠ 00)
Machine Instructions
HLT	HLT	00 000 00X2	Enter STOPPED state; remain there until interupted
HLT	HLT	11 111 1112	Enter STOPPED state; remain there until interupted

See also

• MCS-8
• MCS-4