cspp topic 3

what does the CPU do?

controls and performs instructions

what is a microprocessor?

a silicon chip that modern CPUs are commonly housed on

what are the three key components of the CPU?

ALU, CU, registers

how does the fetch-execute cycle work generally?

computational instructions are retrieved from memory, decoded into recognisable operations, and executed to affect the CPU’s current state

what occurs during the fetch cycle?

• the PC holds the address of the next instruction

• this address is sent through the address bus to memory

• CU issues a Read signal

• memory places the instruction on the data bus

• IR stores this fetched instruction for decoding

what occurs during the decode cycle?

• after fetched, CPU begins to decode instruction

• CU analyses the binary instruction to understand what action the CPU needs to perform

• identifies which parts of the CPU will be involved

• CU sends the necessary control signals to prepare the system for execution

how is the instruction interpreted during the decode cycle?

• CPU examines the fetched binary instruction

• the first few bits of the instruction form opcode, tells the CPU what operation to perform

• remaining bits of the instruction are operands, identify where to get the data

what is opcode?

operation code, tells the CPU what operation to perform

what occurs during the execution cycle?

• once instruction is decoded, CPU carries out the required operation

• CU activates the necessary circuits, or triggers data movement between memory and CPU

• ALU performs computation for arithmetic or logic instructions

• for data transfer instructions, data is moved to or from memory between internal parts of the CPU

what does the ALU do?

receives control signals from the CU during execution phase, takes all inputs from data lines or registers, performs the operation, and sends the result back for storage or further use

what do registers do?

provide fast, temporary storage, during computation

what does the CU do?

• directs the CPU to execute a sequence of instructions in order

• fetches, decodes, and executes each instruction, ensuring the ALU and registers work together

• decodes program instructions and handles logistics for the execution of decoded instructions

what does the IR do?

contains most recent instruction to be fetched

what does the PC do?

tracks the memory address of the next instruction to be executed

what does the MAR do?

contains address of the region of memory to be read or written

what does the MDR do?

contains data fetched from memory or data ready to be written to memory

what is the main difference between the 68008 and 68000 models?

68008 processor has smaller external buses

• has an 8-bit external data bus (16-bit on 68000)

• has a 20-bit external address bus (24-bit for 68000)

describe the internal architecture of 68000 CPU.

• internal registers are 32 bits wide

• internal data buses are 16-bits wide

• 8-bit external data bus

• 20-bit external data bus

what are the D0-D7 registers?

32-bit data registers that store frequently used values / intermediate results

what are the A0-A6 registers?

address registers used as pointer registers in the calculation of operand addresses

what are A7 registers?

address registers used by the processor as a system stack pointer to hold subroutine return addresses etc.

what is CCR?

condition code register, register that contains flags indicating the status of the processor after arithmetic and logical operations

what is a status register?

16 bits (consists of two 8-bit registers),

what is the stack pointer used for?

as a pointer into the system stack

what is the system stack?

a LIFO structure that provides temporary storage of essential processor state

what is the PC?

program counter, keeps track of the address at which the next instruction will be found, incremented to point to the next instruction when the current instruction has been read

what is RTL?

register transfer language, used to describe the operations of a microprocessor as it is executing instructions

how is instruction fetching written in RTL?

1. [MAR] ← [PC]

2. [PC] ← [PC] + 1

3. [MBR] ← [MS([MAR])]

4. [IR] ← [MBR]

5. CU ← [IR(opcode)]

what is an advantage of using C?

you can work at the hardware-software interface and is compiled to assembler (the low level instruction set of the microprocessor)

how does assembly work?

uses easily remembered mnemonics for each instruction, allows memory locations and constants to be given symbolic names

what are advantages of assembly language?

allows memory locations and constants to be given symbolic names, a point in a program can be referred to by its name rather than a numeric address

what are the two aspects to the 68008 instruction set?

instructions, addressing modes

what are instructions in an instruction set?

the commands that tell the processor what operations to perform

what are addressing modes in an instruction set?

the ways in which the processor can access data or memory locations

what are the five categories of instructions in the 68008 instruction set?

1. data movement

2. arithmetic

3. logical

4. branch

5. system control

what form are 68008 assembler instructions written?

operation.datatype source, destination
where the operation can be

• .b (byte)

• .w (word - 2 bytes)

• .l (long word - 4 bytes)

why do arithmetic instructions only operate on integers in the 68008 model?

it does not have hardware floating point support

what are logical instructions?

instructions that perform bit-wise operations on data and include AND, OR, NOT

what are branch instructions used for?

causes the processor to branch (jump/GOTO) the labelled address, can test the state of the CCR bits and branch if a certain condition is met

what are subroutines useful for?

frequently used sections of a program, reduces program size, improves readability, reduces repeated debugging

what happens during JSR (subroutine call)?

saves (pushes) the contents of the PC on the stack, puts start address of subroutine in PC

what happens during RTS (return from subroutine)?

restores (pops) the return address from the stack and puts it in PC, so the stack stores where to return from a subroutine

how does the stack allow for subroutines to call another subroutine?

has multiple return addresses on the stack in this case

what are addressing modes?

how we tell the computer where to find data it needs

where can systems data be located in the 68008 model?

in a data register, within the instruction itself, or in external memory

what does it mean for addressing modes to have expressive power?

they can provide data exactly, specify where data is, or specify how to go about finding data

what are the 5 types of addressing modes in 68008?

1. data or address register direct

2. immediate addressing

3. absolute addressing

4. address register indirect (5 variations)

5. relative addressing

how does data or address register direct work in 68008?

the address of an operand is specified by either a data register or an address register

how does immediate addressing work in 68008?

the operand forms part of the instruction and remains constant throughout the execution of a program

how does absolute addressing work in 68008?

the operand specifies the location in memory explicitly, meaning that no further processing required. does not use a # symbol

why does absolute addressing not allow position independent code?

a program will consistently use the same memory address