Assembly

Question 1

What is an instruction set?

Answer 1

An instruction set is a series of commands typically written in machine code which a CPU is programmed to recognise and respond to. An instruction set can refer to all the instructions for a particular CPU or a subset.

-WhatIs.com

Question 2

Features of the MIPS Processor

Answer 2

The MIPS (Microprocessor without Interlocked Pipelined Stages) is a processor with 32 general registers.

It uses 32-bit instructions to form its instruction set.

32 bit represents a word in MIPS.

Notable for a small number of formats encoding opcodes

It uses a 32x32 bit register file for frequently accessed data.

Question 3

What does MIPS stand for?

Answer 3

Microprocessor without Interlocked Pipelined Stages

Question 4

How are instructions represented in an instruction set?

Answer 4

They are encoded in binary or machine code.

Question 5

How are instructions encoded in MIPS?

Answer 5

They are encoded as 32-bit instruction words.

Question 6

What instruction formats are part of MIPS?

Answer 6

MIPS has three instruction formats,

The I Format: I for immediate

The J Format for Jump Instructions

The R Format for arithmetic or register-based instructions.

Question 7

Define Register.

Answer 7

A register is a small amount of memory in the CPU, it is quickly accessible and used to primarily to perform arithmetic operations in the CPU, storing the results of operations, the numbers on which the calculation is performed with etc. Registers may also have specific functions where some registers cannot be written to and only read from.

Question 8

What are the purposes of registers?

Answer 8

Registers are the fastest and primary form of memory in the computer. They are most often used to perform arithmetic operations as accessing main memory is far slower. Certain register s may have special functions such as the $zero register in MIPS which cannot be rewritten and is always zero.

Question 9

How many general-purpose registers does the MIPS processor have?

Answer 9

32 general registers.

Question 10

What is a register operand?

Answer 10

An operand, in general, refers to the piece of an instruction that specifies what data can be manipulated. As such the term register operand in the context of MIPS means that registers act as operands and the data stored on them

Question 11

What is the MIPS regsiter file?

Answer 11

A register file is an array of the processor registers on a CPU. MIPS uses a 32x32 bit register file.

It is numbered 0 to 31, with 32-bit words.

Question 12

When is the MIPS register file used?

Answer 12

The register file is used for frequently accessed data.

Question 13

What register operand names are used for the assembler?

Answer 13

Assembler names include:

$s0 to $s7 for the saved variables

$t0 to $t7 for the temporary values.

Question 14

What is the MIPS design principle?

Answer 14

Smaller is faster, maximise design with millions of memory locations

Question 15

What are the MIPS general registers?

Answer 15

Question 16

What is register r0?

Answer 16

r0 has the assembly name $zero. It represents 0 and is a read-only register.

It is always zero.

Question 17

What is register r30?

Answer 17

Register r30 has the assembly name $fp, it is the frame pointer.

Question 18

What is register r31?

Answer 18

Register r31 has the assembly name $ra, it is the return address

Question 19

What is register r29?

Answer 19

Register r29 has the assembly name $sp, it is the stack pointer.

Question 20

What is register r28?

Answer 20

Register r1 has the assembly name $gp, it is the global pointer.

Question 21

What is register r1?

Answer 21

Register r1 has the assembly name $at, it is reserved for pseudoinstructions.

Question 22

What are registers r4to r7?

Answer 22

These registers have the names $a0, $a1, etc.. They store arguments

Question 23

What are registers r2 to r3?

Answer 23

These registers have the names $v0, $v1. They store results.

Question 24

What are registers r26 to r27?

Answer 24

These registers have the names $k0, $k1.. They are reserved by the operating system

Question 25

What are registers r23 to r25?

Answer 25

These registers have the names $t8, $t9.. They store temporary which are not saved

Question 26

What are registers r16 to r23?

Answer 26

These registers have the names $s0, $s1, etc.. They store values to be saved for future use.

Question 27

What are registers r8 to r15?

Answer 27

These registers have the names $t0, $t1, etc.. They store temporary values which are not saved.

Question 28

Define assembly language

Answer 28

Assembly language is a programming tool designed to be closer to machine code. The instructions available in assembly are dependent on the machine's instruction set. It has an almost 1:1 mapping with machine code

Question 29

Why was assembly used, and why is it used today?

Answer 29

Assembly was designed to make code a bit easy to understand in early systems, however, small resources meant that the instruction set had to be as efficient as possible. Today, assembly is used each time a compiler is run, used in the inner core of operating systems and to optimise a system. It provides process control overtime cost. Suitable for low-level interactions such as a microcontroller, it helps with performance analysis as to allows observation of the compiler and where the program spending most of its time.

Question 30

What is the format for Arithmetic Operations?

Answer 30

Arithmetic Operations use the R-Format, the instructions require three operands. Operands are a destination, and two sources: **add a, b, c # a = b+c** Here the design principle is simplicity favours regularity • Regularity makes implementation simpler • Simplicity enables higher performance at lower cost

Question 31

Registers Vs Memory

Answer 31

Registers are faster to use than memory, this is in part because using memory requires loads and stores. These are slow instructions. As such compilers should use registers as much as possible, only enter memory for rarely used variables.

Question 32

What is the main memory mostly used for?

Answer 32

Used for composite data. Arrays, structures, dynamic data.

Question 33

How are arithmetic operators applied to memory?

Answer 33

1. Load values from memory into registers 2. Store result from register to memory

Question 34

How is memory addressed?

Answer 34

Memory is byte addressed, i.e. 8-bits.

Question 35

Words in memory?

Answer 35

Words are aligned in memory, this means an address must be a multiple of 4 for 32 bit words.

Question 36

Format for R-Type Instructions

Answer 36

**Instruction fields** * op: operation code (opcode) – for R-type instructions this is always zero * rs: first source register number * rt: second source register number * rd: destination register number * shamt: shift amount (only used in shift operations) * funct: function code (extends opcode) – defines what operation should be run.

Question 37

Format for I (Immediate)-Type Instruction

Answer 37

**Immediate arithmetic and load, store instructions** * op: this time, identifies the instruction * rt: destination or source register number * Constant: –2¹⁵ to +2¹⁵ – 1 * Address: offset added to the base address in rs

Question 38

Features of Immediate Instructions

Answer 38

Constant data is specified in the instruction. As no subtraction immediate instruction, negative numbers are used for subtractions. When designing immediate instructions, make the common case fast. Small constants are common, and immediate operands avoid the load instruction.

Question 39

Format for Jump Instructions

Answer 39

**Jump (j and jal) targets could be anywhere in the text segment** * Encode full address in the instruction **(Pseudo)Direct jump addressing** * Target address = PC31…28 : (address × 4)

Question 40

How do jump instructions work?

Answer 40

The jump instructions load a new value into the PC register, which stores the value of the instruction being executed. This causes the next instruction read from memory to be retrieved from a new location.

Question 41

How do shift operations work in MIPS?

Answer 41

Uses the shamt operand in R-type insructions to specify the number of positions to shift. For shift left operations, shift left and fill with 0 bits, effectively multiplied by 2ⁱ where **i** the number of positions shifted. For shift right operations, shift right and fill with 0 bits. Divides by 2ⁱ unsigned only

Question 42

When should an AND operation be used?

Answer 42

Can be useful to mask bits, select some and erase the rest.

Question 43

Why use OR operations

Answer 43

Can be useful for including bits into a word.

Question 44

How to implement NOT operations.

Answer 44

MIPS doesn't have a NOT instruction but does have a nor instruction. NOR = NOT(OR(a,b))) As such to implement not, nor a word with zero.

Question 45

How are conditional instructions implemented?

Answer 45

There are three instructions used to implement conditional statements, the beq, bne. The beq instruction compares two operands: **beq rs, rt, L1** if (rs == rt) branch to instruction labeled L1; Then the program jumps to instruction L1. The **bne** instruction works similarly except if (rs != rt) branch to instruction labeled L1; then unconditional jump.

Question 46

How are loops implemented in MIPS?

Answer 46

Loops are implemented using j instructions and the beq and bne instructions to end loops.

Question 47

What are the Addressing Modes?

Answer 47

There are 5 addressing modes, Immediate addressing, register addressing, base addressing, PC relative addressing and pseudo-direct addressing.

Question 48

Describe Immediate Addressing

Answer 48

The operand is a constant within the instruction

Question 49

Describe Register Addressing

Answer 49

Where the operand is a register.

Question 50

Describe Base/Displacement Addressing

Answer 50

Where the operand is at the memory location which is the sum of a register and a constant included in the instruction.

Question 51

Describe PC- Relative Addressing

Answer 51

Where the branch address is the sum of the PC and a constant in the instruction

Question 52

Describe Pseudo-direct Addressing

Answer 52

Where the jump address is the 26 bits in the instruction concatenated with the upper bits of the PC.

Question 53

What is the slt instruction?

Answer 53

The slt instruction is a conditional operator. It has three operands, the destination register, and 2 sources. Example: **slt, rd, rs, rt #** if rs \< rt then rd = 1 Often used in conjunction with branch instructions, beq and bne.

Question 54

What is sign extension?

Answer 54

To increase the size of a data item by replicating the high-order sign bit of the original data item in the high order bits of the larger, destination data item.

Question 55

Where is sign extension often used?

Answer 55

Sign extension is used in many applications, in essence whenever a constant is needs to be expanded to a 32-bit word. Most often used in addresses.

Question 56

Requirements for load byte and load halfword

Answer 56

lb rt, offset(rs); lh rt, offset(rs) • Sign extend to 32 bits in rt

Question 57

Requirements for load byte unsigned and load halfword unsigned?

Answer 57

lbu rt, offset(rs) lhu rt, offset(rs) • Zero extend to 32 bits in rt

Question 58

How does store byte and store halfword work?

Answer 58

sb rt, offset(rs) sh rt, offset(rs) • Store just rightmost byte/halfword

Question 59

Why use lui instruction and how

Answer 59

Used to for constants greater than 16 bits. For the occasional 32-bit constant lui rt, constant • Copies 16-bit constant to left 16 bits of rt • Clears right 16 bits of rt to 0

Question 60

Whyt are there no blt, bge etc instructions?

Answer 60

This is because beq and bne are the most common cases, and the hardware is slower for less than comparisons compared to equality so beq and be are used.

Question 61

What are pseudo instructions?

Answer 61

Pseudoinstructions unlike most assembler instruction do not represent machine code in a one to one conversion. They are more similar to macros, at assembly are substituted for real instructions.

Question 62

How is memory structured in MIPS?

Answer 62

Memory is divided into several sections. The lowest level is memory reserved by the OS. Above that is text memory, then static data, then above that is dynamic data and the stack.

Question 63

What is static data?

Answer 63

Static data are global variables, such as: e.g., static variables in C, constant arrays and strings The global pointer is initialized to address allowing ±offsets into this segment

Question 64

What is dynamic data?

Answer 64

Dynamic data is the heap, where user-created memory is stored e.g. malloc in C, new in C++

Question 65

What is the stack, how is it used?

Answer 65

The stack ia automatic (static) data storage. It is used via the following: Local data allocated by callee • e.g. C automatic variables. • Procedure frame $fp (activation record) is used by some compilers to manage stack storage.

Question 66

What is procedure calling?

Answer 66

Procedure calling is simply the act of executing an instruction.

Question 67

What are the steps required for a procedure call?

Answer 67

Steps required: 1. Place parameters in registers 2. Transfer control to procedure (callee) 3. Acquire storage for procedure 4. Perform procedure’s operations 5. Place result in register for caller 6. Return to place of call

Question 68

What is a basic block

Answer 68

A basic block is a sequence of instructions with • No embedded branches (except at the end) • No branch targets (except at the beginning)

Question 69

Why are basic blocks important?

Answer 69

These are important because: • A compiler identifies basic blocks for optimization • An advanced processor can accelerate execution of basic blocks

Question 70

What are the procedure call instructions and how are they used?

Answer 70

Procedure call: jump and link jal ProcedureLabel • Address of the following instruction put in $ra • Jumps to target address • Procedure return: jump register jr $ra • Copies $ra to program counter • Can also be used for computed jumps • e.g., for case/switch statements

Question 71

What is a leaf procedure

Answer 71

A leaf procedure is one that doesn't call any other procedures.

Question 72

What are non-leaf procedures?

Answer 72

Procedures that call other procedures • For nested call, caller needs to save stuff on the stack: • Its return address • Any arguments and temporaries needed after the call • Restore from the stack after the call