131 Week 16 - ARM Assembly serialisation

Question 1

Instruction

Answer 1

The most basic unit of computer processing

Question 2

Assembler

Answer 2

Converts each instruction into a 16/32 bit representation using a well defined translation process.
Each binary instruction the instruction, registers, immediate values.

Question 3

ARM Assembler

Answer 3

There are 2 methods of representing ARM assembly in machine code - ARM T32 and Thumb-2
Thumb-2 is the default.

Question 4

Machine code

Answer 4

Will be made up of binary values
Will have a section to specify
- what instruction is being executed e.g., add, sub
- the input operands
- the output operands

Question 5

ADD in ARM assembly to machine code

Answer 5

First 12 bits - Instruction opcode. Specifies what instruction is being executed.
Next 4 bits - First input register
Next 1 bits - Blank
Next 3 bits - Optional shift
Next 4 bits - Output register
Next 2 bits - Optional shift
Next 2 bits - Optional shift type
Next 4 bits - Second input register
ARM uses 4 bits for registers as there are only 16 registers and 4 bits is enough to represent 0-15, a value for each register.

Question 6

Moore’s law

Answer 6

Moore’s law states that the speed of a CPU doubles every year and the speed of memory doubles every 18 months.
Because memory performance is increasing at a slower rate than the CPU, memory is often a bottleneck.

Question 7

ARM and Thumb instructions

Answer 7

To solve the bottleneck caused by differences in CPU and memory speeds ARM tried to compress code to reduce memory usage, which improved performance.
Thumb was invented in 1995 and uses 16-bit instructions to improve code density and increase performance.
In 2003, Thumb-2 was introduced. Thumb-2 adds some 32-bit instructions as not all instructions suited 16-bit.

Question 8

Thumb-2 example

Answer 8

First 9 bits - opcode
Next 1 bit is the higher bit for input/source register
Next 3 bits is the second input register
Next 2 bits is the remaining 2 bits of the input/source register

Question 9

How does the CPU know if the instruction is a 16 or 32 bit instruction.

Answer 9

If the first 5 bits are 11101, 11110 or 11111, the instruction is 32 bits long. If it starts with any other sequence of 5 bits, it is a 16-bit instruction.