8051 MCU Tutorial Lesson 12: Logical Operation Instructions

Logical operations on accumulator A:

CLRA; clear the value in A to 0, single-cycle single-byte instruction, with the same effect as MOVA, #00H.

CPLA; reverse the value in A bit by bit

RLA; logically shift the value in A to the left

RLCA; logically shift left by adding the carry bit to the value in A

RRA; logically shift the value in A to the right

RRCA; logically shift right by adding the carry bit to the value in A

SWAPA; swap the high and low 4 bits of the value in A.

For example, if (A) = 73H, CPLA is executed as follows:

73H is converted to binary as 01110011.

The bit-by-bit inversion is 10001100, which is 8CH.

RLA sends the 7th bit of the value in (A) to the 0th bit, the 1st bit to the 0th bit, and so on.

For example: The value in A is 68H, and RLA is executed. 68H is converted to binary 01101000, and the shift is performed according to the above figure. 01101000 is converted to 11010000, which is D0H.

RLCA shifts the value in (A) with the carry bit (C).

For example: if the value in A is 68H and the value in C is 1, then RLCA is executed.

After 101101000, the result is 011010001, that is, the value of the carry bit of C becomes 0, and (A) becomes D1H.

I won’t talk much about RRA and RRCA. Please refer to the two examples above and practice on your own.

SWAPA swaps the high and low 4 bits of the value in A.

For example: (A) = 39H, then after executing SWAPA, the value in A is 93H. Why is it swapped like this? Because this is a hexadecimal number, and each hexadecimal digit represents 4 binary bits. Note that if it is like this: (A) = 39, without H behind it, after executing SWAPA, (A) = 93 is not. It needs to be converted into binary and then calculated: 39 is converted into binary 10111, which is 0001. The upper 4 bits of 0111 are 0001, and the lower 4 bits are 0111. After swapping, it is 01110001, which is 71H, or 113.

Exercise, given (A) = 39H, execute the following instructions and write down the results of each step

CPLA

RLA

CLRC

RRCA

SETBC

RLCA

SWAPA

Through the previous learning, we have mastered quite a few instructions. You may be a little bored with these boring instructions. Let's relax and do an experiment.

Experiment 5:

ORG0000H

LJMPSTART

ORG30H

START:

MOVSP,#5FH

MOVA,#80H

LOOP:

MOVP1,A

RLA

LCALLDELAY

LJMPLOOP

delay:

movr7,#255

d1:movr6,#255

d2:nop

nop

nop

nop

djnzr6,d2

djnzr7,d1

ret

END

Let us first write the program into the chip, install it into the experimental board, and see the phenomenon.

What we see is a phenomenon of dark spots flowing. Let us analyze it.

We will analyze the previous ORG0000H, LJMPSTART, ORG30H later. Starting from START, MOVSP, #5FH, this is to initialize the stack. It doesn't matter whether this sentence is in this program, but we are slowly starting to get in touch with formal programming, so I will slowly cultivate the habit for everyone.

MOVA, #80H, sends the number 80H to A. What is it for? I don't know. Read on.

MOVP1, A. Send the value in A to the P1 port. At this time, the value in A is 80H, so the value sent out is 80H, so the value of the P1 port is 80H, which is 10000000B. Through the previous analysis, we should know that at this time, the LED connected to P1.7 is not bright, while the other LEDs are bright, so a "dark spot" is formed. Continue to look at RLA, RLA is to shift the value in A to the left. Calculate, what is the result after the shift? Yes, it is 01H, which is 00000001B. In this way, the LED connected to P1.0 should not be bright, while the others are bright. From the phenomenon, the "dark spot" flows to the back. Then call the delay program, which we are very familiar with, to make this "dark spot" "dark" for a while. Then transfer to LOOP (LJMPLOOP). Please calculate which light should be off below. . . . . Yes, it should be connected to P1.1. The light is not on. This cycle forms the phenomenon of "dark spot flow".

question:

How to achieve highlight flow?

How to change the direction of flow?

Answer:

1. Change the initial value in A to 7FH.

2. Change RLA to RRA.