NotesAtHKU

Computer components

Overview of components

System bus connects CPU, memory, and I/O devices.

The key design concepts of Von Neumann architecture are:

Data and instructions are stored in a single read-write memory.
The contents of memory are addressable by location.
Execution occurs in sequential fashion.

Key terms

Byte: 8 bits
Word: Unit of memory, e.g. 32-bit word
Registers: Small amount of fast storage inside CPU.
Buffer: Temporary storage for data.

Storage inside CPU

Short	Name	Holds
PC	Program Counter	Address of next instruction
IR	Instruction Register	Current instruction `
MAR	Memory Address Register	Address of memory location
MBR	Memory Buffer Register	Data to be written to memory or data read from memory
I/O AR	I/O Address Register	Address of I/O device
I/O BR	I/O Buffer Register	Data to be written to/read from I/O device

Instruction set

Instruction format

Instructions:

Are in 32-bit (4-byte) binary format, and can be one-word or multi-word.
Can be represented by hexadecimal format (hand-assembled). They can also be simplified to assembly.
Instructions are made up of an operator code (code) and some parameters (operands).

Types of operations

Data transfer
Arithmetric
Logical
Control flow
Input / Output
Data conversion

Arithmetric operations treat operands as numbers, and has to consider the sign of operands.

Logical operations treat operands as bit patterns

Course instruction set

The following is our list of instructions, sorted by operator code:

Mnemonic	Assembled	Assembly Syntax	Description	Type
ADD	`00######`	`ADD src1, src2, dst`	Add src1 and src2, store in dst	Arithmetic
SUB	`01######`	`SUB src1, src2, dst`	Subtract src2 from src1, store in dst	Arithmetic
AND	`02######`	`AND src1, src2, dst`	Bitwise AND src1 and src2, store in dst	Logical
OR	`03######`	`OR src1, src2, dst`	Bitwise OR src1 and src2, store in dst	Logical
NOT	`04##00##`	`NOT src1, dst`	Bitwise NOT src1, store in dst	Logical
MOV	`05##00##`	`MOV src1, dst`	Copy src1 to dst	Data transfer
LD	`0600ff## 000000$$`	`LD adr, dst`	Load value from `adr` to `dst`	Data transfer
ST	`07##ff00 000000$$`	`ST src1, adr`	Store value from `src1` to `adr`	Data transfer
BR	`0800ff00 000000$$`	`BR label`	Branch to label Regardless of output	Control flow
BZ	`0801ff00 000000$$`	`BZ label`	Branch if Zero	Control flow
BNZ	`0802ff00 000000$$`	`BNZ label`	Branch if Not Zero	Control flow
HALT	`09000000`	`HLT`	Stops the execution	Control flow
PUSH	`0A##0000`	`PUSH r`	Push `r` to stack (temp store value)	Data transfer
POP	`0B0000##`	`POP r`	Pop from stack to `r` (restore value)	Data transfer
CALL	`0C00ff00 000000$$`	`CALL label`	Call function at label	Control flow
RET	`0D000000`	`RET`	Return from function	Control flow

This table is useful during assembly programming and manual assembly.

Decoding shorthands

This section involve hand-assembling an assembly program.

A simplified overview for completion of assignments:

src, r, dst are a memory registers. In assembly syntax, it should be R1, R2 etc. The assembled equivalent is the number of the register. (e.g. assembly syntax R1 = assembed 01, replace ##)
adr are addresses of memory location. In assembly syntax, it should be P1, P2 etc. The assembled equivalent is the address of memory location. (e.g. assembly syntax P1 replace $$ with address).
label are also addresses, but in assembly syntax, it should be a label.

In a program, each instruction is 4-bytes, so the address of an instruction on line $n$ is $4(n-1)$ (0-indexed). Refer to example below.

There are other ways to specify values in assembly for src, r, dst and adr, described in addressing modes.

Example ADD

0000 0001 0010 0011 represents an addition (0000) of the numbers in memory locations 2 (0010) and 3 (0011) and store the result in memory location 1 (0001).

The instruction might sometimes also be expressed in hex format: 0x00 01 02 03.

Example BNZ

Consider the following set of instructions:

0000H: 01010101
0004H: 0802ff00
0008H: 00000000

The BNZ instruction on line 2 and 3 (recall BNZ is 2-worded) will loop back to line 1 at address (0000H).

Addressing modes

Addressing modes are ways you can specify the address of an operand. Using these methods can reduce the size of program code (as don't need to calculate address explicity), but hardware will be more complicated. What addressing modes a program can use depends on the hardware.

Mode	Notation	Explaination	Advantages	Disadvantages
Immediate	`MOV #5,`	Value specified directly	No memory reference	Limited operand magnitude
Direct	`MOV 10,`	Value in address `10`	Large operand magnitude	Limited address space
Indirect	`MOV (10),`	Value in address specified in value in address `10`	Large address space	Multiple memory ref.
Register	`MOV R,`	Value of R	No memory reference	Limited address space
Register Indirect	`MOV (R),`	Value in address specified in value of R	Large address space	Extra memory ref.
Displacement	`MOV 2(R)`	Value in address specified in value of R offset by 2	Flexibility	Complexity
Stack	`PUSH R1, <>` , `POP <>, R1`	`<>` is the implicit return address stored on the stack	No memory reference	Limited applicability

More on instruction set

Instruction operands

In most applications, instructions either have three, two, one, or zero operands (or addresses). Symbolically, they are represented as:

no. operands	Assembly representation	Interpretation
3	`OP A,B,C`	`A` $\leftarrow$ `B OP C`
2	`OP A,B`	`A` $\leftarrow$ `A OP B`
1	`OP A`	`AC` $\leftarrow$ `AC OP A`
0	`OP`	`T` $\leftarrow$ `(T-1) OP T`

Where AC is accumulator, T is top of stack, and T-1 is the next of stack.

Function calls (procedures / branches)

A procedure consists of multiple instructions that are executed in sequence. Within a procedure, instructions can be given to execute another procedure. For the CPU to know where to go and where to return after the called procedure is done, the return addresses need to be stored, which is done by a stack. The latest return address will be at the top of the stack, and the CPU will pop it when it reaches a return instruction.

Data types

Two types of data types exist: (1) Numeric (integer, floating point) and (2) Non-numeric (character, binary data). Their lengths are typically 8, 16, 32, or 64 bits.

For the MIPS architecture, a family of reduced instruction set computers (RISC), there are 9 basic data types: (1) signed and unsigned bytes, (2) signed and unsigned half-words, (3) signed and unsigned words, (4) double words, (5) single-precision floating point (32 bits), and (6) double-precision floating point (64 bits).

For the ARM architecture, it supports data types of (1) byte (8 bits), (2) half-word ( 16 bits), and (3) word (32 bits) in length. It only provides unsigned integers, nonnegative integers, and two’s complement integers. Floating point hardware is not provided in ARM architecture and must be emulated in software.

Assembly language programming

Assembly language is a low-level programming language that is very instruction set architecture (ISA) specific. Our courses focuses on the following ISA:

Comments are preceded by #
Destination operands are on the right of the operands list
Instructions are case insensitive

Syntax

Each line of assembly language consists of:

label: mnemonic operand1, operand2, ... # comment

label is an optional label that can be used to refer to the instruction later
mnemonic is either a operand or a assembler directive
operandx is the operand(s) for the operation
comment is an optional comment that describes the instruction

You can find the list of mnemonics in the instruction set.

Assembler directive

An assembler directive is a command to the assembler, not an instruction to the CPU. They start with a . and are not executed by the CPU:

Directive	Description
`.data`	Adds the subsequent data to the data segment
`.text`	Adds the subsequent code to the program
`.global NAME`	Makes the label `NAME` visible to other modules
`.space <EXPRESSION>`	Reserves space with the size of `<EXPRESSION>` in bytes, filled with `0`s
`.word value1 [, value2, ...]`	Put the values in successive memory locations, each occupying 4 bytes

Flow control

If else structure

if (a[0] > a[1]): x = a[0];
else: x = a[1];

    .data               # declare start of data segment
a:  .word 1             # a[0] = 1
    .word 2             # a[1] = 2
x:  .word 0             # x = 0
    .text               # declare start of code segment

main:                   # main function
    ld  #a, r8          # set r8 to adress of a[0]
    ld 0(r8), r9        # set r9 to a[0]
    ld 4(r8), r10       # set r10 to a[1] (4 bytes after a[0])
    bgt r9, r10, then   # if a[0] > a[1], goto then
    st r10, x           # else: x = a[1]
    br end

then:
    st r9, x            # x = a[0]
end:
    ret                 # return to caller

For loop structure

a = 0;
for (int i = 0; i < 10; i++) a += i;

    .data                   # declare start of data segment
a:  .word 0                 # initialize a to 0
    .text                   # declare start of code segment

main:                       # main function
    sub r8, r8, r8          # prepare r8 = 0 as the counter
    ld 10, r9               # constant 10 in r9
    ld 1, r10               # constant 1 in r10 for incrementing r8
    sub r11, r11, r11       # use r11 as sum

L:  add r11, r8, r11        # r11 += r8
    add r8, r10, r8         # r8++
    bgt r9, r8, L           # branch to L if r9 (10) > r8 (counter)
    st r11, a               # store the result in a
    ret                     # return to caller

Function calling

Use call to call a function and ret to return from a function. Unlike high-level languages, you must manage the parameters and result of the function yourself.

Specify the input and output parameter registers in the function's comment
Use push and pop to save and restore registers temporarily.

Execution cycle

The cycle is as follows:

Step 1. IAC Instruction address calculation

Address to the next instruction is stored in PC register, which is incremented automatically during execution.

For a two-word instruction, the first word is fetched first, then PC is incremented by 1 word to point to the second word. Then the second word is fetched, and PC is incremented again to point to the next instruction.

Step 2. IF Instruction fetch

The following operations happen during instruction fetch:

MAC \leftarrow PC PC \leftarrow PC + 1 IR \leftarrow mem[MAC]

Step 3. ID Instruction decode

The control unit handles:

Decoding the instruction
Setups up CPU components like ALU at the right time

Step 4. OA Operand address calculation

...

Step 5. OF Operand fetch

If the operands are in registers, data are moved from registers to ALU.

If the operands are in memory, then the instruction would be a two-word instruction. Note that if PC points to the second word of a two-word instruction, after the above process, MBR will contain the address of the second word, not the content. Therefore, another memory read is needed to fetch the content, by:

MAR \leftarrow MBR\\ MBR \leftarrow mem[MAR]

Step 6. EX Execution

The ALU performs the operation specified by the instruction. The result is stored in some temporary register.

Step 7. WB Write back

Similar to operand fetch, if the destination is in register, RF write is performed. If the destination is in memory, then operand address calculation is first performed. Then the data is written to memory.

Interrupt handling

Interruptions are important as:

They improve efficiency.
When an I/O arrives, it may need immediate attention, or data may be lost. e.g. incoming data from a network.
Other programs may also need the CPU’s attention. e.g. on a time-sharing system.

When interruption is required, I/O device sends a signal to the CPU. The CPU will need to remember the current state of the program, and then jump to serve the interrupt. The CPU will then return to the original program and continue execution as if nothing happened.

Interrupt handlers can either be hardware or software

Instructions