Computer Science — AS

Assembly Language (Cambridge International AS Computer Science): Exam Questions

7 questions · 32 marks · ≈ 32 min

Guided practice

15 marksmedium

(a)

The following table shows part of the instruction set for a processor. The processor has two registers, the Accumulator (ACC) and the Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDM`	`#n`	Immediate addressing. Load the number n to ACC
`LDD`	`<address>`	Direct addressing. Load the contents of the location at the given address to ACC
`LDI`	`<address>`	Indirect addressing. The address to be used is at the given address. Load the contents of this second address to ACC
`LDX`	`<address>`	Indexed addressing. Form the address from + the contents of the index register. Copy the contents of this calculated address to ACC
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`ADD`	`#n/Bn/&n`	Add the number n to the ACC
`ADD`	`<address>`	Add the contents of the given address to the ACC
`SUB`	`#n/Bn/&n`	Subtract the number n from the ACC
`SUB`	`<address>`	Subtract the contents of the given address from the ACC
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
`DEC`	`<register>`	Subtract 1 from the contents of the register (ACC or IX)
can be an absolute or a symbolic address # denotes a denary number, e.g. #123 B denotes a binary number, e.g. B01001010 & denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address	Data
`19`	`25`
`20`	`23`
`21`	`2`
`22`	`4`
`23`	`15`
`24`	`50`
`25`	`22`

The current contents of the ACC and IX are shown:

ACC	50
IX	20

Complete the table by writing the content of the ACC and the IX after each set of instructions has run.

	Instructions	ACC content	IX content
1	`LDM #19` `DEC ACC`
2	`LDD 23` `ADD 19`
3	`LDI 25` `INC ACC`
4	`LDR #21` `LDX 2`

Your answer

How did you do?/ 5

23 marksmedium

(a)

The following table shows part of the instruction set for a processor. The processor has two registers, the Accumulator (ACC) and the Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDM`	`#n`	Immediate addressing. Load the number n to ACC
`LDD`	`<address>`	Direct addressing. Load the contents of the location at the given address to ACC
`LDI`	`<address>`	Indirect addressing. The address to be used is at the given address. Load the contents of this second address to ACC
`LDX`	`<address>`	Indexed addressing. Form the address from + the contents of the index register. Copy the contents of this calculated address to ACC
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`ADD`	`#n/Bn/&n`	Add the number n to the ACC
`ADD`	`<address>`	Add the contents of the given address to the ACC
`SUB`	`#n/Bn/&n`	Subtract the number n from the ACC
`SUB`	`<address>`	Subtract the contents of the given address from the ACC
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
`DEC`	`<register>`	Subtract 1 from the contents of the register (ACC or IX)
can be an absolute or a symbolic address # denotes a denary number, e.g. #123 B denotes a binary number, e.g. B01001010 & denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address	Data
50	54
51	55
52	50
53	52
54	100
55	25
56	50

The current contents of the ACC and IX are shown:

ACC	50
IX	45

Complete the table by writing the content of the ACC after each program has run.

	Instructions	ACC content
1	`LDD 50` `ADD #4` `ADD 54`
2	`LDI 53` `DEC ACC` `ADD 56`
3	`LDM #55` `SUB #5`

Your answer

How did you do?/ 3

34 marksmedium

(a)

The following table shows part of the instruction set for a processor. The processor has two registers: the Accumulator (ACC) and an Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDM`	`#n`	Immediate addressing. Load the number n to ACC
`LDD`	`<address>`	Direct addressing. Load the contents of the location at the given address to ACC
`LDI`	`<address>`	Indirect addressing. The address to be used is at the given address. Load the contents of this second address to ACC
`LDX`	`<address>`	Indexed addressing. Form the address from + the contents of the index register. Copy the contents of this calculated address to ACC
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`ADD`	`#n/Bn/&n`	Add the number n to the ACC
`ADD`	`<address>`	Add the contents of the given address to the ACC
`SUB`	`#n/Bn/&n`	Subtract the number n from the ACC
`SUB`	`<address>`	Subtract the contents of the given address from the ACC
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
can be an absolute or a symbolic address # denotes a denary number, e.g. #123 B denotes a binary number, e.g. B01001010 & denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address	Data
19	24
20	2
21	1
22	3
23	5
24	4
25	22

The current contents of the ACC and IX are shown:

ACC	12
IX	1

Complete the table by writing the content of the ACC after each program has run.

Program number	Instructions	ACC content
1	`LDD 20` `ADD #2`
2	`LDX 22`
3	`LDI 25` `INC ACC` `SUB 22`
4	`LDD 19` `LDM #5` `LDM #25`

Your answer

How did you do?/ 4

44 marksmedium

(a)

The following table shows part of the instruction set for a processor. The processor has two registers: the Accumulator (ACC) and an Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDM`	`#n`	Immediate addressing. Load the number n to ACC
`LDD`	`<address>`	Direct addressing. Load the contents of the location at the given address to ACC
`LDI`	`<address>`	Indirect addressing. The address to be used is at the given address. Load the contents of this second address to ACC
`LDX`	`<address>`	Indexed addressing. Form the address from + the contents of the index register. Copy the contents of this calculated address to ACC
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`ADD`	`#n/Bn/&n`	Add the number n to the ACC
`ADD`	`<address>`	Add the contents of the given address to the ACC
`SUB`	`#n/Bn/&n`	Subtract the number n from the ACC
`SUB`	`<address>`	Subtract the contents of the given address from the ACC
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
can be an absolute or a symbolic address # denotes a denary number, e.g. #123 B denotes a binary number, e.g. B01001010 & denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address	Data
10	1
11	3
12	5
13	11
14	10
15	16
16	12

The current contents of the ACC and IX are shown:

ACC	10
IX	0

Complete the table by writing the content of the ACC after each program has run.

Program number	Instructions	ACC content
1	`LDI 15` `SUB #1`
2	`LDD 14` `ADD 11`
3	`LDM #11` `ADD #3` `SUB 16`
4	`LDR #2` `LDX 14` `ADD #2`

Your answer

How did you do?/ 4

54 marksmedium

(a)

The following table shows part of the instruction set for a processor. The processor has two registers: the Accumulator (ACC) and an Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDM`	`#n`	Immediate addressing. Load the number n to ACC
`LDD`	`<address>`	Direct addressing. Load the contents of the location at the given address to ACC
`LDI`	`<address>`	Indirect addressing. The address to be used is at the given address. Load the contents of this second address to ACC
`LDX`	`<address>`	Indexed addressing. Form the address from + the contents of the index register. Copy the contents of this calculated address to ACC
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`ADD`	`#n/Bn/&n`	Add the number n to the ACC
`ADD`	`<address>`	Add the contents of the given address to the ACC
`SUB`	`#n/Bn/&n`	Subtract the number n from the ACC
`SUB`	`<address>`	Subtract the contents of the given address from the ACC
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
can be an absolute or a symbolic address # denotes a denary number, e.g. #123 B denotes a binary number, e.g. B01001010 & denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address	Data
48	51
49	6
50	48
51	50
52	49
53	50
54	6

The current contents of the ACC and IX are shown:

ACC	2
IX	50

Complete the table by writing the content of the ACC after each program has run.

Program number	Instructions	ACC content
1	`LDM #50` `INC ACC` `SUB #1`
2	`LDI 51` `ADD 52`
3	`LDR #2` `LDX 50` `DEC ACC`
4	`LDD 52` `SUB 54` `INC ACC`

Your answer

How did you do?/ 4

67 marksmedium

1 mark

(a)

Identify the purpose of the first pass of a two-pass assembler.

Your answer

6 marks

(b)

The following table shows part of the instruction set for a processor. The processor has two registers, the Accumulator (ACC) and the Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`STO`	`<address>`	Store contents of ACC at the given address
`ADD`	`<address>`	Add the contents of the given address to the ACC
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
`CMP`	`#n`	Compare the contents of ACC with number n
`JPE`	`<address>`	Following a compare instruction, jump to if the compare was True
`OUT`		Output to the screen the character whose ASCII value is stored in ACC
can be an absolute or symbolic address # denotes a denary number, e.g. #123

(i) Give one example of an instruction that belongs to each of the following instruction groups.

Only use the instructions given in the table. Each instruction must have a suitable operand.

Data movement ...........................

Arithmetic operation ...................

Conditional instruction ................

(3 marks)

(ii) The instruction LDR #2 uses immediate addressing.

Give one similarity and one difference between direct addressing and indexed addressing.

(2 marks)

(iii) Identify one other mode of addressing.

(1 mark)

Your answer

How did you do?/ 7

75 marksmedium

4 marks

(a)

The following table shows part of the instruction set for a processor. The processor has two registers, the Accumulator (ACC) and the Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDD`	`<address>`	Direct addressing. Load the contents of the location at the given address to ACC
`LDX`	`<address>`	Indexed addressing. Form the address from + the contents of the index register. Copy the contents of this calculated address to ACC
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`STO`	`<address>`	Store the contents of ACC at the given address
`ADD`	`#n`	Add the denary number n to the ACC
`JMP`	`<address>`	Jump to the given address
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
`CMP`	`<address>`	Compare the contents of ACC with the contents of
`CMI`	`<address>`	Indirect addressing. The address to be used is at the given address. Compare the contents of ACC with the contents of this second address
`JPE`	`<address>`	Following a compare instruction, jump to if the compare was True
`IN`		Key in a character and store its ASCII value in ACC
`OUT`		Output to the screen the character whose ASCII value is stored in ACC
`END`		Return control to the operating system
can be an absolute or a symbolic address # denotes a denary number, e.g. #123

The instructions in the processor’s instruction set can be grouped according to their function.

Identify the instruction group for each of the following opcodes.

IN .............................

ADD .........................

JPE ...........................

CMI ...........................

Your answer

1 mark

(b)

The current contents of main memory and selected values from the ASCII character set are given on page 15.

Trace the program currently in memory using the trace table when the input is ‘1’.

Table with computer instructions, memory addresses, and ASCII values. Includes instruction and output columns, memory values for addresses 100-111.

Your answer

How did you do?/ 5

15 marksmedium

(a)

The following table shows part of the instruction set for a processor. The processor has two registers, the Accumulator (ACC) and the Index Register (IX).

Instruction	Explanation
Opcode	Operand
`LDM`	`#n`	Immediate addressing. Load the number n to ACC
`LDD`	`<address>`	Direct addressing. Load the contents of the location at the given address to ACC
`LDI`	`<address>`	Indirect addressing. The address to be used is at the given address. Load the contents of this second address to ACC
`LDX`	`<address>`	Indexed addressing. Form the address from + the contents of the index register. Copy the contents of this calculated address to ACC
`LDR`	`#n`	Immediate addressing. Load the number n to IX
`ADD`	`#n/Bn/&n`	Add the number n to the ACC
`ADD`	`<address>`	Add the contents of the given address to the ACC
`SUB`	`#n/Bn/&n`	Subtract the number n from the ACC
`SUB`	`<address>`	Subtract the contents of the given address from the ACC
`INC`	`<register>`	Add 1 to the contents of the register (ACC or IX)
`DEC`	`<register>`	Subtract 1 from the contents of the register (ACC or IX)
can be an absolute or a symbolic address # denotes a denary number, e.g. #123 B denotes a binary number, e.g. B01001010 & denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address	Data
`19`	`25`
`20`	`23`
`21`	`2`
`22`	`4`
`23`	`15`
`24`	`50`
`25`	`22`

The current contents of the ACC and IX are shown:

ACC	50
IX	20

Complete the table by writing the content of the ACC and the IX after each set of instructions has run.

	Instructions	ACC content	IX content
1	`LDM #19` `DEC ACC`
2	`LDD 23` `ADD 19`
3	`LDI 25` `INC ACC`
4	`LDR #21` `LDX 2`

Your answer

How did you do?/ 5