Chapter 4 (Section 2) #4

Question 1

Section 1: Fundamental Concepts & Language Types

Question 2

Why is machine code considered the only language a CPU can “use”?

Answer 2

Because it is the only language the hardware is designed to interpret directly as binary, because the hardware is made out of transistors and logic gates that can only process 0s and 1s. Any language that is not machine code needs to be translated before the CPU can understand it.

Question 3

Is machine code machine dependent?

Answer 3

Yes, every different type of computer has its own set of machine code instructions.

Question 4

What is an instruction?

Answer 4

A single operation preformed by the CPU.

Question 5

How does the “length” of an assembly program compare to a high-level language program?

Answer 5

A program in assembly language requires many more instructions than a high-level language to perform the same task.

Question 6

What are opcodes and operands?

Answer 6

Opcodes: Short for operation code. The part of the machine code instruction that identifies what action the CPU will do.
Operand: The part of a machine code instruction that identifies the data to be used by the CPU.

Question 7

Define “Mnemonic” in the context of Assembly.

Answer 7

A short, abbreviated name (like LDD or STO) used in assembly language to represent a specific machine code opcode.

Question 8

What is source code?

Answer 8

A computer program before translation into machine code.

Question 9

Section 2: The Two-Pass Assembler (Pass 1)

Question 10

What is an assembler? What are the different types of assemblers?

Answer 10

A computer program that translates programming code written in assembly language into machine code.
Assemblers can be one-pass or two-pass

Question 11

What do single pass assemblers do (one-pass)?

Answer 11

It puts the machine code instructions straight into the computer memory to be executed.

Question 12

What is the first thing an assembler does in Pass 1 regarding code formatting (in a two-pass assembler)?

Answer 12

It checks the syntax and removes all comments.

Question 13

In Pass 1, what happens when the assembler encounters a symbolic address for the first time (in a two-pass assembler)?

Answer 13

It checks the symbol table; if it’s not there, it adds it to the “symbolic address” column with a corresponding “unknown” address in the address column.

Question 14

What is a “Literal Table”?

Answer 14

A table created during Pass 1 (in a two-pass assembler) if the programmer has used constants in the program.

Question 15

What are macros?

Answer 15

They are a group of instructions given a name (subroutine), can be executed multiple times within a program.
They are written once and called whenever needed.

Question 16

Explain “Macro Expansion” during Pass 1.

Answer 16

The assembler replaces the macro call with the actual group of instructions defined in the macro.

Question 17

What are directives?

Answer 17

commands that provide instructions to the assembler, but do not result in actual machine code instructions (information for assembler).

Question 18

What happens to “Directives” during the first pass?

Answer 18

They are acted upon (e.g., memory is reserved or origin points are set), but they do not generate machine code.

Question 19

Section 3: The Two-Pass Assembler (Pass 2)

Question 20

What is the primary output of Pass 2?

Answer 20

The object code (the actual machine code instructions consisting of opcode and operand).

Question 21

What happens in Pass 2?

Answer 21

1. The computer reads the assembly language program one line at a time.
2. Generate the object code, including the opcode and operand, from the symbol table generated in Pass 1.
3. Forward references are resolved. This is required as some labels may be refered to before their address is known.
4. Symbolic adresses are replaced by absolute references.
5. Save or execute the program.

Question 22

What is the specific role of a “Loader”?

Answer 22

It is a program required to load an object program (produced by a two-pass assembler) into memory so it can be executed.

Question 23

Section 4: Detailed Addressing Modes

Question 24

What is the “formula” for Indexed Addressing?

Answer 24

Effective Address = Operand Address + Contents of Index Register (IX).

Question 25

How does Relative Addressing calculate the target address?

Answer 25

Effective Address = Current Memory Address + Operand value.

Question 26

In Indirect Addressing, if Address 200 contains 50, and Address 50 contains 10, what does "LDI 200" load?

Answer 26

It loads 10 (the contents of the address found at address 200).

Question 27

Which addressing mode is only used in assembly and uses labels?

Answer 27

Symbolic Addressing.

Question 28

Section 5: Data Movement Instructions

Question 29

What instruction loads a literal number directly into the accumulator?

Answer 29

LDM #n (Immediate addressing).

Question 30

How do you load the contents of the Index Register into the Accumulator?

Answer 30

LDR ACC.

Question 31

What does the "LDX" opcode do?

Answer 31

It performs an indexed load: loading the contents of an address (Operand + IX) into the ACC.

Question 32

What instruction returns control to the operating system?

Answer 32

END.

Question 33

Section 6: Arithmetic & Logical Instructions

Question 34

Does "ADD #20" add the value at address 20 or the number 20?

Answer 34

The number 20 (because of the # symbol indicating immediate data).

Question 35

Which registers can the "INC" (Increment) instruction be used on?

Answer 35

The Accumulator (ACC) or the Index Register (IX).

Question 36

Which registers can the "DEC" (Decrement) instruction be used on?

Answer 36

The Accumulator (ACC) or the Index Register (IX).

Question 37

How do you subtract a value at a specific memory location from the ACC?

Answer 37

SUB

.

Question 38

Section 7: Comparison & Control Flow

Question 39

What is the prerequisite for using JPE or JPN?

Answer 39

A CMP (Compare) instruction must have been executed immediately before.

Question 40

What does "JPN" stand for/do?

Answer 40

Jump if Not equal (jumps if the previous comparison was False).

Question 41

What does "JPE" stand for/do?

Answer 41

Jump if Equal (jumps if the previous comparison was True).

Question 42

What is modified when a "Jump" occurs?

Answer 42

The Program Counter's (PC) contents are changed to the specified address, so the next instruction to be executed is the one at that address.

Question 43

How does "CMI" (Compare Indirect) determine what to compare?

Answer 43

It looks at the address in the operand, finds the address stored there, and compares the contents of that second address with the ACC.

Question 44

Section 8: Input/Output and Notations

Question 45

What data type does the "IN" instruction handle?

Answer 45

A single character (storing its ASCII value).

Question 46

Where must a value be located before it can be "OUT" to the screen?

Answer 46

In the Accumulator (ACC).

Question 47

What does the prefix "&" signify in an operand?

Answer 47

A hexadecimal number (e.g., &7B).

Question 48

What does the prefix "B" signify in an operand?

Answer 48

A binary number (e.g., B01000011).

Question 49

Section 9: Programming & Tracing Logic

Question 50

What is the purpose of the "loop:" label in code?

Answer 50

It provides a symbolic target for JMP, JPN, or JPE instructions to return to.

Question 51

What does the "CIR" column in a trace table represent?

Answer 51

The Current Instruction Register (the address of the instruction being executed).

Question 52

Why is a "Symbol Table" essential for the Assembler?

Answer 52

It maps human-readable labels to actual memory addresses so the CPU can find data and jump targets.

Question 53

Section 10: Adressing modes

Question 54

What is Absolute Addressing?

Answer 54

The contents of the memory loaction specified by the operand are used. Ex. LDD 200 would load the value at address 200 into the ACC,

Question 55

What is Direct Addressing?

Answer 55

The contents of the memory loaction specified by the operand are used. Identical to Absolute Addressing.

Question 56

What is Indirect Addressing?

Answer 56

The contents of the memory location that is stored at the address given by the operand are used as the new address to get the data. Ex. If memory location 200 contains the value 20, and memory location 20 contains the value 5, LDI 200 would load 5 into the ACC.

Question 57

What is Indexed Addressing?

Answer 57

The contents of the memory location, that is found by adding the operand (address of the memory location) to the value of the index register (IR), are used. Ex. If the IR contains 4, and memory location 204 contains 17, LDX 200 would load 17 into the ACC (200 + 4, is the new address).

Question 58

What is Immediate Addressing?

Answer 58

The value of the operand is used directly. Ex. LDM #200 loads 200 into the ACC.

Question 59

What is Relative Addressing?

Answer 59

The memory address used is the current memory address added to the operand. Ex. JMR #5 would transfer control to the instruction 5 locations after the current instruction.

Question 60

What is Symbolic Addressing?

Answer 60

Only used in assembly language, where a label is used instead of a memory address. Ex. If memory location MyStore contains 20, LDD MyStore would load 20 in the ACC.

Question 61

What are the purpose of labels?

Answer 61

Make programs easier to alter, as references to absolute addresses don’t need to be manually edited when changes are made.

Question 62

Explain how the instruction ADD 20 can be interperted as either direct or indirect addressing.

Answer 62

Direct addressing: 20 is the address of the data to be used. Indirect addressing: 20 contains the address that contains the data to be used.