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The 8051 Instruction Set

1-10 Atmel 8051 Microcontrollers Hardware Manual

4316A–8051–01/04

MOV A, ENTRY_NUMBER

CALLTABLE

The subroutine “TABLE” would look like this:

TABLE:MOVC A, @A + PC

RET

The table itself immediately follows the RET (return) instruction in Program Memory.

This type of table can have up to 255 entries, numbered 1 through 255. Number 0 can-

not be used, because at the time the MOVC instruction is executed, the PC contains the

address of the RET instruction. An entry numbered 0 would be the RET opcode itself.

1.8 Boolean

Instructions

8051 devices contain a complete Boolean (single-bit) processor. The internal RAM con-

tains 128 addressable bits, and the SFR space can support up to 128 other addressable

bits. All of the port lines are bit-addressable, and each one can be treated as a separate

single-bit port. The instructions that access these bits are not just conditional branches,

but a complete menu of move, set, clear, complement, OR and AND instructions. These

kinds of bit operations are not easily obtained in other architectures with any amount of

byte-oriented software.

The instruction set for the Boolean processor is shown in Table 1-9. All bit accesses are

by direct addressing. Bit addresses 00H through 7FH are in the Lower 128, and bit

addresses 80H through FFH are in SFR space.

Note how easily an internal flag can be moved to a port pin:

MOV C, FLAG

MOV P1.0, C

Table 1-8. Lookup Table Read Instructions

Mnemonic Operation

Execution Time

@ 12MHz (µs)

MOVC A, @A + DPTR Read Pgm Memory at (A + DPTR) 2

MOVC A, @A + PC Read Pgm Memory at (A + PC) 2

Table 1-9. 8051 Boolean Instructions

Mnemonic Operation

Execution Time

@ 12MHz (µs)

ANL C,bit

ANL C,/bit

ORL C,bit

ORL C,/bit

MOV C,bit

MOV bit,C

CLR C

CLR bit

SETB C

SETB bit

CPL C

CPL bit

JC rel

JNC rel

JB bit,rel

JNB bit,rel

JBC bit,rel

C = C AND bit

C = C AND (NOT bit)

C = C OR bit

C = C OR (NOT bit)

C = bit

bit = C

C = 0

bit = 0

C = 1

bit = 1

C = NOT C

bit = NOT bit

Jump if C = 1

Jump if C = 0

Jump if bit = 1

Jump if bit = 0

Jump if bit = 1 ; CLR bit