MELJUN CORTES Memory segmentation, stacks and addressing modes

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Memory Segmentation, Stacks and Addressing Modes

Computer Organization and Assembly Language

Memory Segments

CodeSegment

StackSegment

DataSegment

ExtraSegment

CSSSDSES

SegmentRegisters




Types of Segments

Ø Code Segment. This contains the program or code.

Ø Data Segment. This contains data referenced by almost all instructions and many addressing modes.

Ø Stack Segment. This is for the LIFO stack.

Ø Extra Segment. This is normally for string instructions.




Stack

Ø The stack is implemented in MM of the 8086/8088. It is 64KB long and is organized from a software point of view as 32K words.

Ø The lowest-addressed byte in the current stack is pointed to by the base address in the SS register.




Stack

Ø The SP contains an offset from the value in SS. The address obtained in the SS and SP is the physical address of the top of the stack or TOS (the last storage location in the stack to which data were pushed).

Ø The value of SP is initialized to FFFFH upon start-up of the microcomputer.




Pushing a Data onto the Stack

Ø Whenever a word of data is pushed onto the stack, the high-order 8 bits are placed in the location addressed by SP - 1, and the low-order 8 bits are placed in the location addressed by SP - 2.

Ø SP is then decremented by 2 so that the next word of data is stored in the next available memory location.

Ø Example:PUSH BX




Example 1

Ø Assume the following register contents:

AX =4A30H BP =0008H CS =8000HBX =0010H SP =0040H DS =8000HCX =1800H DI =0006H SS =8002HDX =1234H SI =0040H ES =9000H

Ø After the PUSH BP instruction is executed, what will be the contents of BP, SS, and SP?




Popping a Data onto the Stack

Ø Whenever a word of data is popped from the stack, the low-order 8 bits are removed from the location addressed by SP, and the high-order 8 bits are removed from the location addressed by SP + 1.

Ø SP is then incremented by 2.

Ø Example:POP CX




Example 2



Ø After the PUSH AX instruction is executed, what will be the contents of BP, SS, and SP?




Exercise 1



Ø Given the following program segment:PUSH CXPUSH BXPUSH DL

Ø Assume that instructions are dependent on one another.




Exercise 2



Ø Given the following program segment:POP BXPOP BPPOP SI

Ø Assume that instructions are dependent on one another.




Addressing Modes

Ø Addressing modes refer to the way in which an operand is specified.

Ø Types of Data Addressing Modes:v Register Addressing v Immediate Addressingv Direct Addressingv Register Indirect Addressingv Register Relative Addressing or Base

Addressingv Base-Plus-Index Addressingv Base-Relative-Plus-Index Addressing




Register Addressing

Ø The operand to be accessed is specified as residing in an internal register of the 8086/8088.

Ø Examples:

MOV AX, CX; AX [CX]MOV BX, DX; BX [DX]




Immediate Addressing

Ø The source operand is part of the instruction instead of the contents of a register. Typically, immediate operands represent constant data.

Ø Examples:

MOV AL, 15H; AL 15HMOV AX, 1A3F; AX 1A3FH




Direct Addressing

Ø The location following the instruction opcode holds an effective memory address (EA) instead of data. This EA is the 16-bit offset of the storage location specified by the current value of the DS register.

Example 1:

MOV AX, BETA

Example 2:

MOV AX, LIST




Register Indirect Addressing

Ø Similar to direct addressing but this time, the EA resides in either a base register (BX, BP) or index register (SI, DI) within the 8086/8088.

Example 1:

MOV AX, [BX]

Example 2:

MOV AX, [SI]

Example 3:

MOV CX, [BP]




Register Relative Addressing or Base Addressing

Ø The physical address of the operand is obtained by adding a direct or indirect displacement to the contents of either BX or BP and the current value in DS or SS, respectively.

Example 1:

MOV AX, [BX + 1000H]

Example 2:

MOV AL, [BX] + BETA

Example 3:

MOV DI, SET[BX]




Base-Plus-Index Addressing

Ø This is used to transfer a byte or word between a register and the memory location indicated by the sum of a base register and an index register.

Example 1:

MOV AX, [BX + SI]

Example 2:

MOV [BX + DI], SP




Base-Relative-Plus-Index Addressing

Ø This is used to transfer a byte or word between a register and the memory location addressed by a base register and an index register plus a displacement.

Example 1:

MOV AX, [BX + SI + 0100H];

Example 2:

MOV AX, FILE[BX + DI];




Exercise

Ø Identify the addressing mode of the following instructions:

1. ADD AX, FADEH2. CMP FADE, AX3. INC DH4. ADC [BP+1800H], BX5. AND AGAIN[BP+SI], DS6. OR DX, [DI]7. ADC AX, [BP+SI]

Technology

MELJUN CORTES Memory segmentation, stacks and addressing modes