Addressing Modes and the MOV Instruction
Most assembly language instructions require operands to be processed. An operand address provides the location, where the data to be processed is stored. Some instructions do not require an operand, whereas some other instructions may require one, two or three operands.
When an instruction requires two operands, the first operand is generally the destination, which contains data in a register or memory location and the second operand is the source. Source contains either the data to be delivered (immediate addressing) or the address (in register or memory) of the data. Generally, the source data remains unaltered after the operation.
The three basic modes of addressing are:
In this addressing mode, a register contains the operand. Depending upon the instruction, the register may be the first operand, the second operand or both.
MOV DX, TAX_RATE ; Register in first operand MOV COUNT, CX ; Register in second operand MOV EAX, EBX ; Both the operands are in registers
As processing data between registers does not involve memory, it provides fastest processing of data.
An immediate operand has a constant value or an expression. When an instruction with two operands uses immediate addressing, the first operand may be a register or memory location, and the second operand is an immediate constant. The first operand defines the length of the data.
BYTE_VALUE DB 150 ; A byte value is defined WORD_VALUE DW 300 ; A word value is defined ADD BYTE_VALUE, 65 ; An immediate operand 65 is added MOV AX, 45H ; Immediate constant 45H is transferred to AX
Direct Memory Addressing
When operands are specified in memory addressing mode, direct access to main memory, usually to the data segment, is required. This way of addressing results in slower processing of data. To locate the exact location of data in memory, we need the segment start address, which is typically found in the DS register and an offset value. This offset value is also called effective address.
In direct addressing mode, the offset value is specified directly as part of the instruction, usually indicated by the variable name. The assembler calculates the offset value and maintains a symbol table, which stores the offset values of all the variables used in the program.
In direct memory addressing, one of the operands refers to a memory location and the other operand references a register.
ADD BYTE_VALUE, DL ; Adds the register in the memory location MOV BX, WORD_VALUE ; Operand from the memory is added to register
This addressing mode uses the arithmetic operators to modify an address. For example, look at the following definitions that define tables of data:
BYTE_TABLE DB 14, 15, 22, 45 ; Tables of bytes WORD_TABLE DW 134, 345, 564, 123 ; Tables of words
The following operations access data from the tables in the memory into registers:
MOV CL, BYTE_TABLE ; Gets the 3rd element of the BYTE_TABLE MOV CL, BYTE_TABLE + 2 ; Gets the 3rd element of the BYTE_TABLE MOV CX, WORD_TABLE ; Gets the 4th element of the WORD_TABLE MOV CX, WORD_TABLE + 3 ; Gets the 4th element of the WORD_TABLE
Indirect Memory Addressing
This addressing mode utilizes the computer's ability of Segment:Offset addressing. Generally, the base registers EBX, EBP (or BX, BP) and the index registers (DI, SI), coded within square brackets for memory references, are used for this purpose.
Indirect addressing is generally used for variables containing several elements like, arrays. Starting address of the array is stored in, say, the EBX register.
The following code snippet shows how to access different elements of the variable.
MY_TABLE TIMES 10 DW 0 ; Allocates 10 words (2 bytes) each initialized to 0 MOV EBX, [MY_TABLE] ; Effective Address of MY_TABLE in EBX MOV [EBX], 110 ; MY_TABLE = 110 ADD EBX, 2 ; EBX = EBX +2 MOV [EBX], 123 ; MY_TABLE = 123
The MOV Instruction
We have already used the MOV instruction that is used for moving data from one storage space to another. The MOV instruction takes two operands.
Syntax of the MOV instruction is:
MOV destination, source
The MOV instruction may have one of the following five forms:
MOV register, register MOV register, immediate MOV memory, immediate MOV register, memory MOV memory, register
Please note that:
Both the operands in MOV operation should be of same size
The value of source operand remains unchanged
The MOV instruction causes ambiguity at times. For example, look at the statements:
MOV EBX, [MY_TABLE] ; Effective Address of MY_TABLE in EBX MOV [EBX], 110 ; MY_TABLE = 110
It is not clear whether you want to move a byte equivalent or word equivalent of the number 110. In such cases, it is wise to use a type specifier.
Following table shows some of the common type specifiers:
|Type Specifier||Bytes addressed|
The following program illustrates some of the concepts discussed above. It stores a name 'Zara Ali' in the data section of the memory, then changes its value to another name 'Nuha Ali' programmatically and displays both the names.
section .text global _start ;must be declared for linker (ld) _start: ;tell linker entry point ;writing the name 'Zara Ali' mov edx,9 ;message length mov ecx, name ;message to write mov ebx,1 ;file descriptor (stdout) mov eax,4 ;system call number (sys_write) int 0x80 ;call kernel mov [name], dword 'Nuha' ; Changed the name to Nuha Ali ;writing the name 'Nuha Ali' mov edx,8 ;message length mov ecx,name ;message to write mov ebx,1 ;file descriptor (stdout) mov eax,4 ;system call number (sys_write) int 0x80 ;call kernel mov eax,1 ;system call number (sys_exit) int 0x80 ;call kernel section .data name db 'Zara Ali '
When the above code is compiled and executed, it produces the following result:
Zara Ali Nuha Ali