ASM coding for addressing mode, Lecture notes of Engineering

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Addressing Modes

Assembly Language

• computer language between machine

language and high-level language.

• General format.
• [label:] mnemonic [operand] [, operand] […] [; comment]
• START: MOV A,#0FF ;’START” IS A LABEL.

Mnemonic.field

• mnemonic instruction : ADD, MOV, DIV

• assembler directives: ORG, EQU, DB

operand field.

• contain the address or data used by the

instruction.

• Operand field dependent on the operation.

Some operation have no operand.

Special Assembler Symbol

• For the register-specific addressing

modes. - A, R0, DPTR, PC, & C

SET C

INC DPTR

• Dollar sign ($) – to refer to current value of

the location counter.

JNB TI,$ ;can also be written as HERE: JNB TI,HERE

8051 DATA ADDRESSING MODES

• assembly language instructions

• consist of an operation mnemonic and between zero and three operands separated by commas.
• Where two operand instructions occur, the destination is specified first, followed by the source operand.

• opcode dest_operand source_operand

Addressing Modes

• The first three modes provide access to

the internal RAM and hardware register

space.

• thus can occur in both the source and destination operand fields.

Immediate Addressing

• Data in the operand field.
• Data are preceded with # prefix.
  • The immediate value is a maximum of 8-bits.
  • All bits in the high-byte must be the same (00H or FFH) - MOV A,#0FEH
  • One exception, when dealing with the DPTR register it can be 16-bits. - MOV DPTR, #2000H ; Load the value 2000H into the DPTR register

Direct Addressing

• can access any on-chip variable or hardware

register.

• Memory address (00H to 7FH)
• SFR address (80H to 0FFH)
• Predefined symbol may be used for SFR address

• MOV A,30H

• MOV A, SBUF ; same as MOV A,99H
  • the value to be stored in memory is obtained by directly retrieving it from another memory location

All I/O ports and special function registers are assigned addresses between 80H and 0FFH.

External Direct

• MOVX A,@DPTR : read

• MOVX @DPTR,A : write

– it is used to access external memory

– only two commands, utilize DPTR.

– DPTR is loaded with the address of external

memory that you wish to read or write.

External Indirect

• MOVX @R0,A

– used in small projects with very small amount

of external RAM

– value of @R0 can only be 00h through FFh

thus limited to 256 bytes of External RAM.

Bit Address

• Accessing bit-addressable location

– Data memory (00H to 7FH)

– SFR’s (80H to 0FFH)

• Ways to specify
  • Explicitly giving the address
    • SETB OE7H
  • Using the dot operator between byte address and bit position
    • SETB ACC.
  • Using predefined assembler symbol.
    • JNB SBUF,$

Code Address

• Give in the form of a label

• Used in the operand field for the jump

instructions.

HERE: *

SJMP HERE