Summary of knowledge about 51 pseudo instructions

1. DS --- reserved storage area command

Format: [label:] DS expression value

Its function is to define a storage area starting from a specified address for use by the source program.

The number of storage cells reserved for the storage area is determined by the value of the expression.

TMP: DS 1

Reserve 1 storage unit (byte) starting from the address labeled TEP.

2. BIT - define bit command

Format: Character Name BIT Bit Address

Its function is used to define the bit address for the character name.

SPK BIT P3.7

After definition, SPK is allowed to replace P3.7 in instructions.

3. USING instruction

The USING directive tells the assembler which set of 8051 working registers to use.

Format: USING expression (The value must be 0-3, the default value is 0.)

USING 0

Use group 0 of working registers.

4. SEGMENT instruction

The SEGMENT directive is used to declare a relocation segment and an optional relocation type.

Format: relocation segment name SEGMENT segment type [relocation type]

Among them, the "relocation segment name" is used to indicate the declared segment.

The "segment type" is used to specify the memory address space in which the declared segment will be located.

The available segment types are CODE, XDATA, DATA, IDATA, and BIT.

STACK_SEG SEGMENT IDATA

DATA_SEG SEGMENT DATA 

5. RSEG - Relocation segment selection instruction

The relocation segment selection instruction is RSEG, which is used to select a previously defined relocation segment as the current segment.

Format: RSEG segment name

The segment name must be a relocation segment that has been declared previously.

DATA_SEG SEGMENT DATA ; declare a relocated DATA segment

RSEG DATA_SEG ; select the previously declared relocated DATA segment as the current segment

6. Absolute segment selection instruction

CSEG - Absolute Code Segment

DSEG - Internal absolute data segment

XSEG－－－External absolute data segment

ISEG - Internal indirect addressing data segment

BSEG - absolute bit addressing data segment

Format:

CSEG [AT absolute address expression]

DSEG [AT absolute address expression]

XSEG [AT absolute address expression]

ISEG [AT absolute address expression]

BSEG [AT absolute address expression]

The brackets are optional and are used to specify the base address of the current absolute segment.

CSEG AT 0000H

AJMP MAIN

7. ORG directive

The ORG instruction is used to change the assembler's counter, thereby setting a new program starting address.

Format: ORG expression

The expression must be an absolute or simple relocation expression.

ORG 0000H

AJMP MAIN

Set the starting address of the MAIN program to 0000H.

8. END instruction

The END instruction is used to control the end of the assembly. There must be an END instruction at the end of each assembly program.

And the END instruction can only appear once.

9. EQU directive

The EQU directive is used to assign a value or register name to a specified symbolic name.

Format: symbol name EQU expression  

or symbol name EQU register name

The expression must be a simple relocation expression.

The character name assigned by the EQU instruction can be used as a data address, code address, bit address or

Use it directly as an immediate value.

LIMIT EQU 1200

COUNT EQU R5

10. DATA instruction

The DATA directive is used to assign an internal RAM address to the specified symbol name.

Format: symbol name DATA expression

The value of the numeric expression is between 0 and 255, and the expression must be a simple relocation expression.

PORT1 DATA 40H

11. DB instruction

The DB directive initializes code space with the bytes containing the value of the expression.

Format: [label:] DB expression table

The expression can contain symbols, strings, or expressions. Each item is separated by commas, and strings should be enclosed in quotation marks.

The label in brackets is optional. If used, the value of the label will be the address of the first byte in the expression table.

The DB instruction must be within the CODE section, otherwise an error will occur.

TABLE:

DB 0C0H, 0F9H, 0A4H

TABLE 1:

DB " WEINA "