STC89C53RC datasheet(136/165 Pages) STC

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Mnemonic Field

Intruction mnemonics or assembler directives go into mnemonic field, which follows the label field. Examples of

instruction mnemonics are ADD, MOV, DIV, or INC. Examples of assembler directives are ORG, EQU, or DB.

Operand Field

The operand field follows the mnemonic field. This field contains the address or data used by the instruction. A

label may be used to represent the address of the data, or a symbol may be used to represent a data constant. The

possibilities for the operand field are largely dependent on the operation. Some operations have no operand (e.g.,

the RET instruction), while others allow for multiple operands separated by commas. Indeed, the possibilties for

the operand field are numberous, and we shall elaborate on these at length. But first, the comment field.

Comment Field

Remarks to clarify the program go into comment field at the end of each line. Comments must begin with a

semicolon (;). Each lines may be comment lines by beginning them with a semicolon. Subroutines and large

sections of a program generally begin with a comment block—serveral lines of comments that explain the general

properties of the section of software that follows.

Special Assembler Symbols

Special assembler symbols are used for the register-specific addressing modes. These include A, R0 through

R7, DPTR, PC, C and AB. In addition, a dollar sign ($) can be used to refer to the current value of the location

counter. Some examples follow.

SETB

INC

DPTR

JNB

TI , $

The last instruction above makes effective use of ASM51's location counter to avoid using a label. It could also be

written as

HERE:

JNB

TI , HERE

Indirect Address

For certain instructions, the operand field may specify a register that contains the address of the data. The

commercial "at" sign (@) indicates address indirection and may only be used with R0, R1, the DPTR, or the PC,

depending on the instruction. For example,

ADD

A , @R0

MOVC A , @A+PC

The first instruction above retrieves a byte of data from internal RAM at the address specified in R0. The second

instruction retrieves a byte of data from external code memory at the address formed by adding the contents of

the accumulator to the program counter. Note that the value of the program counter, when the add takes place, is

the address of the instruction following MOVC. For both instruction above, the value retrieved is placed into the

accumulator.

Immediate Data

Instructions using immediate addressing provide data in the operand field that become part of the instruction.

Immediate data are preceded with a pound sign (#). For example,

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