STC89LE53RC datasheet(145/165 Pages) STC

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To create a 1000-byte buffer in external RAM starting at 4000H, the following directives could be used:

XSTART

EQU

4000H

XLENGTH

EQU

1000

XSEG

XSTART

XBUFFER:

DS XLENGTH

This buffer could be cleared with the following instruction sequence:

MOV

DPTR, #XBUFFER

LOOP:

CLR

MOVX

@DPTR, A

INC

DPTR

MOV

A, DPL

CJNE

A, #LOW (XBUFFER + XLENGTH + 1), LOOP

MOV

A, DPH

CJNE

A, #HIGH (XBUFFER + XLENGTH + 1), LOOP

(continue)

This is an excellent example of a powerful use of ASM51's operators and assemble-time expressions. Since an

instruction does not exist to compare the data pointer with an immediate value, the operation must be fabricated

from available instructions. Two compares are required, one each for the high- and low-bytes of the DPTR.

Furthermore, the compare-and-jump-if-not-equal instruction works only with the accumulator or a register, so

the data pointer bytes must be moved into the accumulator before the CJNE instruction. The loop terminates only

when the data pointer has reached XBUFFER + LENGTH + 1. (The "+1" is needed because the data pointer is

incremented after the last MOVX instruction.)

DBIT

The format for the DBIT (define bit) directive is,

[label:]

DBIT

expression

The DBIT directive reserves space in bit units. It can be used only in a BIT segment. The expression must be

a valid assemble-time expression with no forward references. When the DBIT statement is encountered in a

program, the location counter of the current (BIT) segment is incremented by the value of the expression. Note

that in a BIT segment, the basic unit of the location counter is bits rather than bytes. The following directives

creat three flags in a absolute bit segment:

BSEG

;BIT SEGMENT (ABSOLUTE)

KEFLAG:

DBIT

;KEYBOARD STATUS

PRFLAG:

DBIT

;PRINTER STATUS

DKFLAG:

DBIT

;DISK STATUS

Since an address is not specified with BSEG in the example above, the address of the flags defined by DBIT could

be determined (if one wishes to to so) by examining the symbol table in the .LST or .M51 files. If the definitions

above were the first use of BSEG, then KBFLAG would be at bit address 00H (bit 0 of byte address 20H). If other

bits were defined previously using BSEG, then the definitions above would follow the last bit defined.

DB (Define Byte)

The format for the DB (define byte) directive is,

[label:]

expression [, expression] […]

The DB directive initializes code memory with byte values. Since it is used to actually place data constants in

code memory, a CODE segment must be active. The expression list is a series of one or more byte values (each of

which may be an expression) separated by commas.

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