[ASM] Lab2

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Assembly Language - Lab (2)

AgendaWhat is Assembly Language?Computer OrganizationMain memoryMemory Modes:

Real Mode (8086) Protected Mode (80386)

What is Assembly Language? Assembly Language is a low-level (machine‐level) programming

language that uses mnemonics instead of numeric codes (0’s, 1’s) to simplify programming.

For example, the instruction in machine language which means copy the content from AX register to BX register is: 8B D8

While same instruction written in assembly is: mov BX, AX

Each statement in assembly code has a one-to-one relationship with machine language instructions, in other words each statement corresponds to a single machine code instruction.

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To run a program written in assembly language, we should have a converter (or translator) which converts these labels and mnemonics to their corresponding machine codes in 0’s and 1’s. This converter is called an assembler.

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Machine CodeAssemblerAssembly Code

To run a program written in a high level language we should

have a converter called …..?

What is Assembly Language?

Computer Organization

Main Memory

CPU

R2 R3R1

ALUControl unit

R4

Main Memory It is the place to store data (and instructions) temporarily. Each location (byte) in memory has content (value) and a unique

label (address). Often, memory is used in larger chunks than single bytes. As shown

below:

Main Memory

Byte

Memory Modes:Real Mode (8086) – 16 Bit Registers

CPU 8086

Register Size 16 bits

Main Memory 1 MByte

Program Segment 64 k

(2¿¿20𝑏𝑦𝑡𝑒)¿


4 General Purpose AX : Accumulator BX : Base CX: Count DX: Data

4 Index and Pointers DI: Destination Index SI : Source Index BP: Base Pointer SP : Stack pointer

4 Segments registers DS: Data Segment. CS: Code Segment SS: Stack Segment ES: Extra Segment

Instruction Pointers IP: Instruction Pointer

Flags ZF , SF, OF, CF...


4 General PurposeEach of these registers could be decomposed into two 8‐bit registers.

AH and AL are dependent on AX. Changing AX’s value will change AH and AL values and vice versa.


Index and Pointers (SI, DI)They are often used as pointers to memory items, but can be used for other purposes as the general‐purpose registers. They cannot be decomposed into 8‐bit registers.


Index and Pointers (BP, SP)They are used to point to data in the stack and are called the Base Pointer and Stack Pointer, respectively.

StackBP

SP


4 Segments registers (CS, DS, SS, ES)They keep the starting address of memory chunk used for different parts of a program.

CS stands for Code Segment, DS for Data Segment, SS for Stack Segment, and ES for Extra Segment. ES is used as a temporary segment register.


Instruction Pointers (IP)This register is used with the CS register to keep track of the address of the next instruction to be executed by the CPU.

Flags (ZF , SF, OF, CF...)The FLAGS register stores important information about the results of the last executed operation. This information is stored as individual bits in this register. For example, there is a specific bit called the Zero flag (Z flag). This Z flag is 1 if the result of the last operation was zero otherwise Z flag is set to zero. Not all instructions modify the bits in FLAGS.


As shown before, memory in real mode (8086) is limited to only one megabyte (220 bytes).

Valid address range is from (in hex) 00000 to FFFFF. These addresses require a 20‐bit number.

Will it fit in the segment registers?


A program is often divided into 3 segments, which are Code, Data, and Stack segments where its starting address is stored in segment registers. And each of these segments must begins on a paragraph boundary (i.e. its address is divisible by 16). Therefore, the starting address of any segment always begins with four 0‐bits.

In addition, an offset register is used to address memory locations within each segment. The size of each segment is 64KBat most.

Why?


To find the physical address (20‐bit) from segment‐offset pair, use the following relation:

16 * segment register + offset register

Multiplying by 16 is equivalent to left shifting the binary value 4 times. This done to return the four 0 bits which not stored physically in the segment register.

Why do we multiply by

16?


Examples:1. Segment Register: 047C

Offset Register: 0048Physical address: 047C0 + 0048 = 04808

2. Segment Register: 047DOffset Register: 0038Physical address: 047D0 + 0038 = 04808

And we can also get the same address when we add 047E0 with 0028.

Same address!


Disadvantages:1. A single segment can only contain 64K of memory (the upper

limit of the 16‐bit offset register). 2. Each byte in memory does not have a unique segmented

address.

Memory Modes:Protected Mode (80386) – 32 Bit Registers

CPU 80386

Register Size 32 bits

Main Memory 4 Gbyte

Program Segment 4 Gbyte

(2¿¿32𝑏𝑦𝑡𝑒)¿


In 386 CPU, registers become 32‐bit wide except segment registers (selectors) remain 16‐bits as they are.

Two new 16‐bit segment registers are also added, FS and GS. This extension made the single segment size up to 4GB.


So how does a segment register store the 32‐bit address of memory? Segment register is interpreted differently in protected mode. It is

interpreted as an index into a descriptor table than a register stores the starting address of the segment.

Descriptor table: is a table containing physical addresses of all segments beside some other information about these segments. It is stored in memory and its location is stored in a special register. (local or global)


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16 bits CS Register

32 bits Address of the CS in Memory

Main Memory

Descriptor Table


As explained before, this mode allows the program segment to be of size 4GB, so only one segment of only one program can take the whole memory size to its own?


For that problem protected mode uses a technique called virtual memory.

The basic idea of virtual memory system is to only keep the data and code in physical memory that programs are currently using.

Other data and code are stored temporarily on disk until they are needed again.


Furthermore, segments can be divided into smaller 4K‐sized units called pages, so each segment can be divided to 220 pages.

The virtual memory system works with pages now instead of segments.

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Questions?

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Thank you

Education

[ASM] Lab2