Atmel C51 Handbuch PDF herunterladen (Seite 15)

The 8051 Instruction Set

1-13 Atmel 8051 Microcontrollers Hardware Manual

4316E–8051–01/07

The RLA instruction converts the index number (0 through 4) to an even number on the

range 0 through 8, because each entry in the jump table is 2 bytes long:

JUMP_TABLE:

AJMP CASE_0

AJMP CASE_1

AJMP CASE_2

AJMP CASE_3

AJMP CASE_4

Table 1-11 shows a single “CALL addr” instruction, but there are two of them -LCALL

and ACALL -which differ in the format in which the subroutine address is given to the

CPU. CALL is a generic mnemonic which can be used if the programmer does not care

which way the address is encoded.

The LCALL instruction uses the 16-bit address format, and the subroutine can be any-

where in the 64K Program Memory space. The ACALL instruction uses the 11-bit format,

and the subroutine must be in the same 2K block as the instruction following the ACALL.

In any case the programmer specifies the subroutine address to the assembler in the

same way: as a label or as a 16-bit constant. The assembler will put the address into the

correct format for the given instructions.

Subroutines should end a RET instruction, which returns execution following the CALL.

RETI is used to return from an interrupt service routine. The only difference between RET

and RETI is that RETI tells the interrupt control system that the interrupt in progress is

done. If there is no interrupt in progress at the time RETI is executed, then the RETI is

functionally identical to RET.

Table 1-12 shows the list of conditional jumps available to the Atmel 8051 user. All of

these jumps specify the destination address by the relative offset method, and so are

limited to a jump distance of -128 to + 127 bytes from the instruction following the condi-

tional jump instruction. Important to note, however, the user specifies to the assembler

the actual destination address the same way as the other jumps: as a label or a 16-bit

constant.

There is no Zero bit in the PSW. The JZ and JNZ instructions test the Accumulator data

for that condition.

The DJNZ instruction (Decrement and Jump if Not Zero) is for loop control. To execute a

loop N times, load a counter byte with N and terminate the loop with DJNZ to the beginning

of the loop, as shown below for N = 10:

MOV COUNTER, # 10

LOOP:(begin loop)

Table 1-12. Conditional Jumps in Atmel 8051 Devices

Mnemonic Operation Addressing Modes

Execution Time

@ 12MHz (µs)

DIR IND REG IMM

JZ rel Jump if A = 0 Accumulator only 2

JNZ rel Jump if A ≠ 0 Accumulator only 2

DJNZ <byte>,rel

Decrement and jump if

not zero

X X 2

CJNZ A,<byte>,rel Jump if A = <byte> X X 2

CJNE <byte>,#data,rel Jump if <byte> = #data X X 2

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 115 116

Kommentare zu diesen Handbüchern

Keine Kommentare

Atmel C51 Handbuch Seite 15

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Nein Atmel C51