Question

For each of these addressing modes

• Indexed

• register

For each

• Provide a motivated context in C well suited to its use. (hint: think about variable lifetime)

• Indicate whether the encoding of this addressing mode requires an extension word, and why.

Answer 1

addressing modes are all about how do we address the data that we operate on?

a) Indexed addressing - In C, we have arrays - is a continuous allocation of homogenous data types. The machine code for arrays can be represented using the Indexed addressing mode, we can randomly access data at any index.

for egin MOV RI, BX, IDX index registes Base Register ALU + EA RI Tebbet Memory

coming to the encoding of an indexed addressing instruction, words are required to the opcode, destination address, base address and the index address.

b) Register addressing mode - in this addressing mode, both the operands are registers. In C, we often perform arithmetic and logical operations. So, internally the CPU uses the registers to store the intermediate results of these arithmetic and logical operations.

Why the CPU use register? -- registers are storage devices that are very closely located to the processor and the communication between the processor and registers is super fast.

for eg: a = b+c

now this line of C code is internally converted to

mov &b, R4 -- move the data in b to R4

add &c, R4 -- add the data in c to R4

mov R4, &a -- move the data in R4 to a

coming to the encoding of register addressing mode, words are required for the opcode, and two operands.