STC89LE52RC datasheet(142/165 Pages) STC

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ASSEMBLER DIRECTIVES

Assembler directives are instructions to the assembler program. They are not assembly language instructions

executable by the target microprocessor. However, they are placed in the mnemonic field of the program. With the

exception of DB and DW, they have no direct effect on the contents of memory.

ASM51 provides several catagories of directives:

Assembler state control (ORG, END, USING)

Symbol definition (SEGMENT, EQU, SET, DATA, IDATA, XDATA, BIT, CODE)

Storage initialization/reservation (DS, DBIT, DB, DW)

Program linkage (PUBLIC, EXTRN,NAME)

Segment selection (RSEG, CSEG, DSEG, ISEG, ESEG, XSEG)

•

Each assembler directive is presented below, ordered by catagory.

Assembler State Control

ORG (Set Origin)

The format for the ORG (set origin) directive is

ORG

expression

The ORG directive alters the location counter to set a new program origin for statements that follow. A label is

not permitted. Two examples follow.

ORG

100H

;SET LOCATION COUNTER TO 100H

ORG

($ + 1000H) AND 0F00H

;SET TO NEXT 4K BOUNDARY

The ORG directive can be used in any segment type. If the current segment is absolute, the value will be an

absolute address in the current segment. If a relocatable segment is active, the value of the ORG expression is

treated as an offset from the base address of the current instance of the segment.

End

The format of the END directive is

END

END should be the last statement in the source file. No label is permitted and nothing beyond the END statement

is processed by the assembler.

Using

The format of the END directive is

USING expression

This directive informs ASM51 of the currently active register bank. Subsequent uses of the predefined symbolic

the following sequence:

USING 3

PUSH

AR7

USING 1

PUSH

AR7

The first push above assembles to PUSH 1FH (R7 in bank 3), whereas the second push assembles to PUSH 0FH

(R7 in bank 1).

Note that USING does not actually switch register banks; it only informs ASM51 of the active bank.

Executing 8051 instructions is the only way to switch register banks. This is illustrated by modifying the example

above as follows:

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