The CPU Revision Typical machine code instructions Using op-codes and operands Symbolic addressing. Conditional and unconditional branches

The CPU The Central Processing Unit is one of the main parts of a computer It consists of; 1. Control Unit (CU) 2. The Arithmetic Logic Unit (ALU) 3. Registers

Structure of the CPU Arithmetic Logic Unit Accumulator Control Unit Program Counter Instruction Register

Parts of the CPU 1. Control Unit: The manager of the CPU, decodes instructions and controls the operations done by the ALU 2. Arithmetic Logic Unit: Arithmetic and logic operations are carried out. 3. Instruction register: Stores a copy of the current instruction being executed 4. program counter: Stores the address of the next instruction in the program to be executed 5. Bus: Transfer of data

Assembly Language The CPU uses what is known as Assembly Language Assembly Language is made up of op- codes and operands Instructions in assembly language are rather simple

Assembly Language Operations A few examples

Moving Data Set a register (a temporary location in the CPU) to a fixed value STO 10 This will store a result (found in the accumulator) to a memory location, which in this case is 10. Move data from a memory location to a register, or vice versa. This is done to obtain the data to perform a computation on it later, or to store the result of a computation. mov ah, 09 This will move the constant number 9 to the register ah. Read and write data from hardware devices INP ah, 145 Receives input from the input device with the code 145 and stores it into register ah.

Computing Data Add, subtract, multiply, or divide the values of two registers, placing the result in a register ADD AX, BX Add the contents found in the register AX with the contents in register BX and store the result in register AX. Perform bitwise operations, taking the conjunction / disjunction (and/or) of corresponding bits in a pair of registers, or the negation (not) of each bit in a register OR AX, BX Executes a bit-wise OR between the two registers and stores the result in BX. Compare two values in registers (for example, to see if one is less, or if they are equal) CMP AX, BX Compares the values by subtracting the BX from AX, the result is not stored.

Program Flow Jump to another location in the program and execute instructions there JMP dest This will force the flow of the program to jump to the destination specified as an argument. Jump to another location if a certain condition holds JNZ dest This will jump to the destination if the accumulator is not Zero. For more info:

Using Op-Codes (mnemonics) Op-Codes are used instead of binary Op - Codes are short words normally made up of 3 characters When used they perform a function Op-Codes are easier to remember by programmers, since their name refers to what they will perform

Op – Code Examples Op.code (binary)MnemonicFunction 0000LDA LOAD Accumulator with contents of specified address. 0001STA STore Accumulator contents in specified address. 0010ADD ADD contents of the specified address to the accumulator. 0010SUB SUBtract contents of specified address from the accumulator. 0110JPU JumP Unconditionally to the specified address 0111JAZ Jumps to the specified address if Accumulator is Zero. 1001JAL Jumps to specified address if Accumulator < zero. 1010JAG Jumps to the specified address if the Accumulator > zero. 1111HLT Stop the program

Addressing With Assembly Language, we have three ways in which the computer can find locations from memory; 1. Absolute Addressing 2. Relative Addressing 3. Symbolic Addressing

Absolute Addressing When we use Assembly Language different memory locations are used In some cases we can actually specify which memory location we wish to use (example, LDA 10 is specifying that the accumulator should be filled with the contents found in memory location 10) LDA 10

Relative Addressing This is indicated by specifying the distance from another address In this case it is called the base address, hence the exact memory location is not given (example, MOV CX, [BX+4] is specifying that the contents of Register C must be moved in the location which is found 4 locations off register B) Bx Bx+1Bx+2Bx+3Bx+4 MOV CX,[BX+4] Note: BX is the base address

Symbolic Addressing A label can be given to a memory location in assembly language Instead of referring to the memory location by the location a label is used. The memory location 106 can be assigned a label Num1, and from then on, whenever location 106 is required, instead of calling memory location 106, Num1 is called, LDA Num LDA NUM1 NUM1

Conditional & Unconditional branches The program counter (PC) found in the CPU points to the next instruction to be fetched If we change the PC, instructions can be executed in a different order not in a sequence In order to use jump instructions, labels must be used. Labels can be created by simply specifying a label name and adding a colon at the end of the label. For example: label1: mov ax,5 mov bx,3

Unconditional Jump This type of branching instructs the processor to jump to a label without any conditions. The command is JMP When the JMP command is encountered, the flow of the program will resume from the specified label; in this case lab1. mov ax, 5 jmp lab1 add ax, 4 lda 10 lab1: sub ax, 3

… In the program,; 1. As soon as the command jmp lab1 is encountered, the program will jump to the label 2. The program will continue from the label 3. The commands add ax,4 and lda 10 will be skipped 4. sub ax,3 will be worked next. As already mentioned, an unconditional branch does not have any conditions, so when the jump command is encountered it will simply skip to the label.

Conditional Branching Conditional branching allows the program to skip to another location (through the use of labels) only if it satisfies a condition In order to use this type of branching, a comparison must be done first First we carry out the comparison, and then we jump to a label if the comparison satisfies the jump’s criteria

Assembly cmp ax, bx jg isgreater : ; block 1 (else part) jmp after isgreater: : ; block 2 (then part) after: : ; after if 1.The registers ax and bx are compared with each other using the command CMP 2.Then the command jg (jump if greater) is used to jump to the label isgreater, if ax is greater than bx, otherwise it will jump to label after 3.The actual comparison is carried out using the CMP command, and then different jump statements can be used.

Different Jumps InstructionMeaning jg Jump if greater jge Jump if greater or equal jl Jump if less jle Jump if less or equal je Jump if equal jne Jump if not equal jc Jump if carry flag is set