STC89LE53RC datasheet(144/165 Pages) STC

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The DATA, IDATA, XDATA, BIT, and CODE directives assign addresses of the corresponding segment

type to a symbol. These directives are not essential. A similar effect can be achieved using the EQU directive; if

used, however, they evoke powerful type-checking by ASM51. Consider the following two directives and four

instructions:

FLAG1

EQU

05H

FLAG2

BIT

05H

SETB

FLAG1

SETB

FLAG2

MOV

FLAG1, #0

MOV

FLAG2, #0

The use of FLAG2 in the last instruction in this sequence will generate a "data segment address expected" error

message from ASM51. Since FLAG2 is defined as a bit address (using the BIT directive), it can be used in a set

bit instruction, but it cannot be used in a move byte instruction. Hence, the error. Even though FLAG1 represents

the same value (05H), it was defined using EQU and does not have an associated address space. This is not an

advantage of EQU, but rather, a disadvantage. By properly defining address symbols for use in a specific memory

space (using the directives BIT, DATA, XDATA,ect.), the programmer takes advantage of ASM51's powerful

type-checking and avoids bugs from the misuse of symbols.

Storage Initialization/Reservation

The storage initialization and reservation directives initialize and reserve space in either word, byte, or bit units.

The space reserved starts at the location indicated by the current value of the location counter in the currently

active segment. These directives may be preceded by a label. The storage initialization/reservation directives are

described below.

DS (Define Storage)

The format for the DS (define storage) directive is

[label:]

expression

The DS directive reserves space in byte units. It can be used in any segment type except BIT. The expression

must be a valid assemble-time expression with no forward references and no relocatable or external references.

When a DS statement is encountered in a program, the location counter of the current segment is incremented by

the value of the expression. The sum of the location counter and the specified expression should not exceed the

limitations of the current address space.

The following statement create a 40-byte buffer in the internal data segment:

DSEG

30H

;PUT IN DATA SEGMENT (ABSOLUTE, INTERNAL)

LENGTH

EQU

BUFFER:

LENGRH

;40 BYTES RESERVED

The label BUFFER represents the address of the first location of reserved memory. For this example, the buffer

begins at address 30H because "AT 30H" is specified with DSEG. The buffer could be cleared using the following

instruction sequence:

MOV

R7, #LENGTH

MOV

R0, #BUFFER

LOOP:

MOV

@R0, #0

DJNZ

R7, LOOP

(continue)

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