MCS-51 assembly language program format

The general format of an MCS-51 assembly language program

* Unlike 8086, there is no distinction between code segment and data segment, and all addresses are unified.

* A program consists of command lines or comment lines. Each command line can only contain one command.

* “Label:” can be used before an instruction to indicate the instruction address for easy reference in the instruction; a label is a symbolic name that starts with a letter.

* Comment lines should start with a “;”; “;” can also be used to separate additional comments after instructions.

* The program is located by the pseudo instruction ORG XXXXH. The 16-bit address in ORG is the absolute address to be written into the program memory in the future. The ORG in the program should be arranged from small to large addresses. The program segments located by ORG can be discontinuous. The assembler will fill 00H, i.e. NOP, in the empty addresses between segments. Therefore, the last instruction of each ORG segment should generally be a transfer instruction.

* Data is described by pseudo instruction DB (byte) or DW (double byte). The value should start with a digit 0-9; the suffix description: B-binary, D-decimal, H-hexadecimal. Decimal D can be omitted.

* The EQU directive can define an equivalent field so that a symbolic name can be used to represent an operand, for example:

aa EQU 30H

bb EQU #30H

MOV A,aa; aa is equivalent to "30H", the instruction is MOV A, 30H

MOV R0,bb; bb is equivalent to "#30H", the instruction is MOV R0,#30H