COE 205 - 3 Computer Organization & Assembly Language

1. COE 205 - 3Computer Organization & Assembly Language Introduction Machine Language

2. Machine Language • Computers only understand programs in their own machine language. A machine language program written for one type of computers, say a Sun-Ultra 20 machine wont work with another type of computers say an IBM-compatible PC. • It is impractical to ask programmers to write their programs in machine language. Thus, programmers write their programs in HLL and/or assembly language • Each command of the program is called an instruction (a command that instructs the computer what to do) • Computers only deal with binary data, hence the instructions must be in binary format (0s and 1s) • The set of all instructions (in binary form) makes up the computer's machine language • Hence, the program has to be translated to the computer's machine language before it is stored in the memory • Compilers translate HLL instructions to machine languages • Assembler translate assembly instructions to machine languages COE 205 Dr.Talal AlKharobi

3. Micro-Operations • The computer breaks down machine instructions into smaller operations that are called micro-operations • These micro-operations are performed by the computer automatically (i.e. the programmer does not specify them) • Example of micro-operations for an instruction that add two operands are • Fetching the operands, • Add the operands • Writing back the result to the memory COE 205 Dr.Talal AlKharobi

4. Instruction Fields • Machine language instructions usually are made up of several fields. Each field specifies different information for the computer. The major two fields are: • Opcode field: stands for operation code and it specifies the particular operation that is to be performed. Each operation has its unique opcode and may take the computer several micro-operations to accomplish. • Operands fields: specify where to get the source and destination operands for the operation specified by the opcode. The source/destination of operands can be the memory or one of the general-purpose registers. COE 205 Dr.Talal AlKharobi

5. Assembly instructions and machine code. • This table shows a number of assembly instructions and their corresponding machine code. • The first column shows the address at which the instruction is stored, • The second column shows the machine code for the instruction • The third column shows the corresponding assembly instruction. COE 205 Dr.Talal AlKharobi

6. Machine instructions • As can be see from the table, instruction sizes are not the same • Some instructions have 2 bytes, some 3 and some 4 bytes • For example, you can see that the instruction MOV AX, 1 is stored at address 0005. Since the size of this instruction is 3 bytes, the next instruction will be stored at address 0005+3=0008 • As you can see, for the first seven instructions the first byte encodes the opcode and also the code for the register. It can be observed that the least significant 2 bits are encoding the register • The last two bytes encode the constant. Since the register is 16 bits, the constant size is also 16 bits and this is why 2 bytes are allocated COE 205 Dr.Talal AlKharobi

7. Machine instructions • You can observe that when both the operands are registers that the instruction size is 2 bytes • Note the difference between the machine code for the instructions MOV AX, 1 and ADD AX, 1. • Also, note that for the instruction ADD AX, i two bytes are allocated to store the address of variable i which is in this example 0000. In 8086 machines, an address requires 2 bytes. • In order for one to completely understand the mapping between assembly instructions and machine code, it is required to know the various instructions formats and the syntax and semantics of each format. COE 205 Dr.Talal AlKharobi