Addresing modes
- 2. INDEX Addressing Mode. Types of Addressing Modes. 1. Implied Mode. 2. Auto Increment/decrement Mode. 3. Immediate Mode. 4. Indirect Addressing Mode. Instruction Format. Types of Instruction Format. 1. Memory Reference Instructions. 2. Register Reference Instructions. 3. I/O Reference Instructions.
- 3. Addressing Modes Every instruction format contain an opcode i.e operation code which is required to be implemented on some data stored in memory or register. Addressing Modes gives assembly lang. programmer the flexibility to design a program which are more effective w.r.t number of instruction.
- 4. Types of addressing modes Implied Mode :- Implied addressing is an addressing mode which specifies no address at all. EX. CMA - Complement the content of accumulator • Auto Increment/Decrement Mode :- It helps to increment the value of register by 1 is called Auto Increment. And if it is reduced by 1 is called Auto Decrement.
- 5. Immediate Mode In this instead of address to be specified in instruction either the operand or the value of operand is been specified.
- 6. Direct Addressing Mode In this mode the address of the data is specified in the instruction. The data will be in memory. In this addressing mode, the program instructions and data can be stored in different memory. Ex: LDA 5 (Location) Load accumulator with the value available at location 5.
- 7. Indirect Addressing Mode In indirect addressing mode, the instruction specifies the name of the register in which the address of the data is available. Here the data will be in memory and the address will be in the register pair.
- 8. Instruction Format It consist of binary code 0 and 1 group together. It instruct the computer to perform a specific operation. It usually divide into 2 parts :- Operation code – It ia a group of bits that defines operation. Operand – The address of a value contained in a variable on which opcode can be implemented.
- 9. Types of Instruction Format There are 3 types of Instruction Format as follows : I/O Instruction Format. Memory Reference Instruction Format. Register Reference Instruction Format.
- 10. In an instruction format: First 12 bits (0-11) specify an address. Next 3 bits specify operation code (opcode). Left most bit specify the addressing mode I I = 0 for direct address I = 1 for indirect address
- 11. Memory Reference Instructions first 12 bits (0-11) specify an address. The 3 bits utilizes for recognizing the operation code . The last bit i.e I =0 indicate Direct Address. I = 1 indicate Indirect Address. •
- 12. Register Reference Instructions First 12 bits (0-11) specify the register operation. The next three bits equals to 111 specify opcode. The last mode bit of the instruction is 0.
- 13. The instruction helps to carry micro-operation on value in register are :- 1. CLA – Clear AC 2. CLE – Clear content of reg. E 3. CMA – Complement AC 4. SPA – Skip if Positive in AC 5. SNZ – Skip if not Zero
- 14. I/O Reference Instructions First 12 bits (0-11) specify the I/O operation. The next three bits equals to 111 specify opcode. The last mode bit of the instruction is 1.
- 15. There are 25 basic computer instruction some are as follows :- 1. LDA – Load to accumulator 2. STA – Store to accumulator 3. BSA – Branch Save Address 4. ISZ – Increment & skip if zero 5. BUN – Branch unconditing