STC89C53RC datasheet(137/165 Pages) STC

137 / 165 page

CONSTANT

EQU

100

MOV

A , #0FEH

ORL

40H , #CONSTANT

All immediate data operations (except MOV DPTR,#data) require eight bits of data. The immediate data are

evaluated as a 16-bit constant, and then the low-byte is used. All bits in the high-byte must be the same (00H or

FFH) or the error message "value will not fit in a byte" is generated. For example, the following instructions are

syntactically correct:

MOV

A , #0FF00H

MOV

A , #00FFH

But the following two instructions generate error messages:

MOV

A , #0FE00H

MOV

A , #01FFH

If signed decimal notation is used, constants from -256 to +255 may also be used. For example, the following

two instructions are equivalent (and syntactically correct):

MOV

A , #-256

MOV

A , #0FF00H

Both instructions above put 00H into accumulator A.

Data Address

Many instructions access memory locations using direct addressing and require an on-chip data memory address

(00H to 7FH) or an SFR address (80H to 0FFH) in the operand field. Predefined symbols may be used for the

SFR addresses. For example,

MOV

A , 45H

MOV

A , SBUF

;SAME AS MOV A, 99H

Bit Address

One of the most powerful features of the 8051 is the ability to access individual bits without the need for masking

operations on bytes. Instructions accessing bit-addressable locations must provide a bit address in internal data

memory (00h to 7FH) or a bit address in the SFRs (80H to 0FFH).

There are three ways to specify a bit address in an instruction: (a) explicitly by giving the address, (b) using

the dot operator between the byte address and the bit position, and (c) using a predefined assembler symbol. Some

examples follow.

SETB

0E7H

;EXPLICIT BIT ADDRESS

SETB

ACC.7

;DOT OPERATOR (SAME AS ABOVE)

JNB

TI , $

;"TI" IS A PRE-DEFINED SYMBOL

JNB

99H , $

;(SAME AS ABOVE)

Code Address

A code address is used in the operand field for jump instructions, including relative jumps (SJMP and conditional

jumps), absolute jumps and calls (ACALL, AJMP), and long jumps and calls (LJMP, LCALL).

The code address is usually given in the form of a label.

ASM51 will determine the correct code address and insert into the instruction the correct 8-bit signed offset,

11-bit page address, or 16-bit long address, as appropriate.

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