Week 4

Question 1

What do machine code instructions contain?

Answer 1

• Some operation (op-code) such as add, jump etc.

- Necessary auxiliary info (where to get the operands, store results, where to jump to)

Question 2

Where are instructions held?

Answer 2

In memory (as consecutive addresses) as one or more words.

Question 3

What is the op-code?

Answer 3

It’s part of the operation word. It tells the control unit (CU) which operation this is e.g. ADD, MULTIPLY etc

Question 4

How do we prevent instructions from becoming too long?

Answer 4

Restrictions may be imposed on where a particular instruction can be accessed.

Question 5

What are addressing modes in the CPU design?

Answer 5

They specify where an operand for an instruction is to be found or a result should be stored. There are 5 common examples:

1) immediate, e.g. ADD 2
2) Absolute, e.g. gives the numeric address of the memory location involved
3) Register indirect, e.g. tells the control unit to go to the register you specify and fully from memory the value correlating to the address
4) Indexed addressing, e.g. take the base address (memory address) and add to the number in the register.

Question 6

What 3 things does Sigma 16 consist of?

Answer 6

It consists of:

• 16 registers (R0…R15 where R0 is always 0)
• the Program Counter
• 16-bit memory locations (16-bit memory locations with addresses 0000_16 to ffff_16)

Question 7

Where are operands located?

Answer 7

In any memory location or register file register.

Question 8

What is the effective address?

Answer 8

Where the operand is stored (either in memory location or reg file reg).

Question 9

What is Sigma-16?

Answer 9

Virtual CPU (runs as software, not hardware), everything is 16-bits.

Question 10

What is the op-field in Sigma 16?

Answer 10

The most significant 4-bits of the operation word (which is 16-bits long) is called the op-field.

Question 11

What addressing modes are used in Sigma-16?

Answer 11

Register addressing and indexed addressing

Question 12

What are the three Sigma 16 instruction formats?

Answer 12

RRR (Register Register Register)
RX
X

Question 13

how many op codes are there in RRR instruction format?

Answer 13

14:
\+ - x %
compare less than
compare equals
compare greater than
bitwise boolean: inv, and, or, xor
logic shifts: shift left, shift right
trap

Question 14

Are all numbers in RRR 16 bit twos-complement or unsigned?

Answer 14

2’s complement

Question 15

Which addressing mode does RRR instruction use?

Answer 15

register addressing

e.g. ADD R_x,R_y,R_z

Question 16

What is assembly language?

Answer 16

A list of assembly language statements (gets translated into machine code instruction) and occasionally assembler directives (does not get translated to MC, it’s rarely used). It’s simply an instruction to the assembly program to do something.

Question 17

How are assembly language statements constructed?

Answer 17

Label Mnemonic Operands ;Comment

• Label is chosen by the programmer. If there’s no label, make a space.
• Mnemonic: short name for the instruction
• Comment: optional, starts with ;
• for an RRR, the operand field always lists the 3 registers Rd,Ra,Rb (no spaces).

Question 18

How do we denote a hexadecimal number, e.g. 100_16?

Answer 18

$100

Question 19

What does the DATA directive (type of assembler directive) do?

Answer 19

Allows data to be preset in specific memory.

e.g.
x DATA $3000 ;Comment
Gives the label x to the next available memory location and initialises it to $3000.

It’s now possible for other instructions to use the label x as a variable.

Question 20

How many RX Instructions are there?

Answer 20

It has 6 instructions.

Question 21

How are RX instructions composed?

Answer 21

op code, d, a and b.
d and a are needed to specify registers.
b is not. We put a value in the b field (1,2,3) to differentiate between the RX instructions. This is the extension of the op-code.
op code is F (1111)

Question 22

What are the six RX instructions?

Answer 22

LEA Load effective address - loads constants into the register file
LOAD - copies data from memory
STORE
JUMP False
JUMP True
Jump and link

Question 23

What do the RX parameters stand for?

Answer 23

X - memory address/effective address

R - register

Question 24

What is a destination register

Answer 24

d field

Question 25

How are RX instructions comprised?

Answer 25

LOAD Rd,x[Ra]

Load instruction where the content of Ra is added to x and copies the data into Rd.

Question 26

what op-field value does an X format instruction have?

Answer 26

E base 16 (i.e. 1110)

Question 27

What is the main difference between the OW in RX and X format?

Answer 27

RX needs two operation words

X only needs one operation word (no destination register needed).

Question 28

What is the one X instruction?

Answer 28

JUMP

Question 29

What’s the difference between Load and LEA?

Answer 29

Load loads the CONTENT inside the address to register R1, e.g.
LOAD R1, 1024[R0] ; loads the content of register 1024 to register R1

LEA loads the register with a constant. e.g.
LEA R1, 1024[R0] ; loads the value 1024 into register R1

Question 30

What does cmplt R1,R2,R3 do?

Answer 30

Sets R1 as 1 (i.e. true) if R2

Question 31

What does JUMPF R1,x[R2] do?

Answer 31

If R1 is 0, we jump to R2+x. Otherwise do nothing.

Question 32

What’s the definition of source code?

Answer 32

Code that is written by humans (e.g. Java or assembly language) and has not yet been converted to machine code (by a compiler or assembler).

Question 33

What’s the definition of object programs?

Answer 33

Programs that have been turned into machine code. This can be thought of the output of a compiler or assembler. It’s a list of binary words.

Question 34

What do the assembler and compiler do?

Answer 34

They generate necessary content for all memory locations required for program or data.

Question 35

What is a memory image?

Answer 35

It’s a list of binary words and their associated addresses, which can be stored on disk and loaded into primary memory when the object program is to be run.

It will be loaded potentially in blocks, in an increasing list of addresses.

Question 36

What is the start address?

Answer 36

It’s the first instruction that’s supposed to be executed when the program is run. The start address will be loaded into the program counter.