STC89LE53RC datasheet(141/165 Pages) STC

141 / 165 page

So, the assembled instructions are equal to

MOV

A, #0FFH

Even though expressions evaluate to 16-bit results (i.e., 0FFFFH), in the examples above only the low-order

eight bits are used, since the instruction is a move byte operation. The result is not considered too big in this case,

because as signed numbers the 16-bit value FFFFH and the 8-bit value FFH are the same (-1).

Expression Examples

The following are examples of expressions and the values that result:

Expression

Result

'B' - 'A'

0001H

8/3

0002H

155 MOD 2

0001H

4 * 4

0010H

8 AND 7

0000H

NOT 1

FFFEH

'A' SHL 8

4100H

LOW 65535

00FFH

(8 + 1) * 2

0012H

5 EQ 4

0000H

'A' LT 'B'

FFFFH

3 <= 3

FFFFHss

A practical example that illustrates a common operation for timer initialization follows: Put -500 into Timer 1

registers TH1 and TL1. In using the HIGH and LOW operators, a good approach is

VALUE

EQU

-500

MOV

TH1, #HIGH VALUE

MOV

TL1, #LOW VALUE

The assembler converts -500 to the corresponding 16-bit value (FE0CH); then the HIGH and LOW operators

extract the high (FEH) and low (0CH) bytes. as appropriate for each MOV instruction.

Operator Precedence

The precedence of expression operators from highest to lowest is

( )

HIGH LOW

* / MOD SHL SHR

+ -

EQ NE LT LE GT GE = < > < <= > >=

NOT

AND

OR XOR

When operators of the same precedence are used, they are evaluated left to right.

Examples:

Expression

Value

HIGH ( 'A' SHL 8)

0041H

HIGH 'A' SHL 8

0000H

NOT 'A' - 1

FFBFH

'A' OR 'A' SHL 8

4141H

141

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