Day 2

Question 1

which register holds the instruction to be decoded and executed?

Answer 1

The IR register

Question 2

• what is o.p?*
• how many o.p are there?*

Answer 2

• a short cut of operation code*
• 5 bits - 32 operaion codes*

Question 3

Explain immidiate addressing

• why is it called that way?*
• why we can’t always use that?*

Answer 3

• The instruction contains the operand itself(data) rather than an address which points to some data.
• Such an operand is called an immediate operand because it is automatically fetched from memory at the same time the instruction itself is fetched; hence it is immediately available for use.
• 10 bits are saved for the register, thus if the data is of size > 2^17 immediate addressing can’t be done.

Question 4

Explain direct addressing

Answer 4

• Specifying an operand in memory by giving its full address.
• the instruction will always access exactly the
  same memory location. So while the value can change, the location cannot.
• Direct addressing can only be used to access global variables whose address is known at compile time.

Question 5

what is indirect addressing

Answer 5

An address which specifies not the operand but the address of the operand.

Question 6

what is direct register addressing?

which registers are used?

Answer 6

same as direct addressing but specifies a register instead of a memory location. Usually using a register is faster than using the memory.

It uses the 32 temporary use registers

Question 7

What is indirect register addressing?

Answer 7

• The operand specified comes from memory or goes to memory
• The address is contained in a register
• When an address is used this way it is called a pointer
• we can now reference memory without having its full address in the instruction
• It can use different memory words on different memory executions of the instruction.

Question 8

For each instruction determine its addressing type

Answer 8

First three instructions: (note: # means constant)

• First operand - register direct
• Second operand - immediate

Fourth instruction:

• First operand - register direct
• Second operand - register indirect

Fifth instruction:

• First operand - register direct
• Second operand - immediate
• Sixth instruction:*
• First and second operand - register direct

Question 9

what is register index addressing

Answer 9

• reference memory words at a known offset from a register.
• register + const

Question 10

what is relative addressing?

Answer 10

• op address = address of PC register + const

Question 11

• Which instruction use only ra and c1.*
• That is, the instruction is of the form:*

opcode ra c1.

why is there no need for a base register?

Answer 11

• ldr, str, lar*
• There is no need for a base register because the base register is implicit - PC register.*
• ldr ra, PC, c1*
• str ra, PC, c1*
• lar ra, PC, c1*

Question 12

What is the structure of an instruction?

Answer 12

• 32 bits in total
• 5 bits for instruction code
• next 5 bits - ra register to which we can add
  
  • c1 constant - up to 22 bits
• next 5 bits - rb register to which we can add
  
  • c2 constant - up to 17 bits
• next 5 bits - rc register to which we can add
  
  • c3 constant - up to 12 bits

The ra,rb,rc registers describe one of the 32 registers