In Register Addressing the operand is a register and the registers are used in addressing operation, while in Pseudo-Direct Addressing the operand depends on the program counter PC in which the jump address concatenated with the upper bits of PC.

 

However, the direct method is not generally used at present, because it is a traditional method. It's simple, it just needs one memory reference, and it doesn't need additional calculations.

Register adressing

Register addressing where the operand is a register

Addressing Mode is the way in which the operand is addressed in an instruction word before being actually executed.

Relative Address Mode :

In this mode, the Effective Address (EA) of the operand is calculated by adding the content of the CPU register and the address part of the instruction word. The effective address is calculated by adding displacement (immediate value given in the instruction) and the register value. The address part of the instruction is usually a signed number, either positive or negative. The effective address thus calculated is relative to the address of the next instruction.

Relative Addressing Mode can be further classified into three types –

2. Direct Address Mode :

In Direct Address Mode, the effective address of the operand is equal to the address part of the instruction, i.e. the address part of the instruction indicates the memory location containing the operand.

Pseudo-direct Addressing is the memory address which (mostly) embedded in the instructions.

addressing is specifically used for J-type instructions , j and jal. The instruction format is 6 bits of opcode and 26 bits for the immediate value (target).

Therefore , the new effective address will always be a word-aligned and we can never have a target address of a jump instruction with the two bits anything other than 0 0 and creates a complete 32-bit address. Since the upper 4 bits of the PC are used, this constrains the jump target to anywhere within the current 256 MB block of code (1/16 of the total 4 GB address space). To jump anywhere within the 4 GB space, the R-type instructions jr and jalr are used , where the complete 32 - bit target address is specified in a register.

In direct addressing mode, address field in the instruction contains the effective address of the operand and no intermediate memory access is required. Now a days it is rarely used.

2. Indirect Addressing Mode:

In Indirect addressing mode, address field in the instruction contains the memory location or register where effective address of operand is present. It requires two memory access. It is further classified into two categories:Register Indirect, and Memory Indirect

* The number of registers is limited since only a few bits are reserved to select a register.

In Register Addressing the operand is a register and the registers are used in addressing operation

‎‏Pseudo-direct Addressing is the memory address which (mostly) embedded in the instructions.

 Pseudo-Direct Addressing the operand depends on the program counter PC in which the jump address concatenated with the upper bits of PC.

* Pseudo-Direct addressing is specifically used for J-type instructions , j and jal. The instruction format is 6 bits of opcode and 26 bits for the immediate value (target).

In this addressing mode, a register contains the operand. Depending upon the instruction, the register may be the first operand, the second operand or both. As processing data between registers does not involve memory, it provides fastest processing of data.

In Pseudo-Direct addressing :

 the effective address is calculated by taking the upper 4 bits of the Program Counter(PC) , concatenated to the 26 bit immediate value , and the lower two bits are 00.