20 KiB
20 KiB
- #CT213 - Computer Systems & Organisation
- Previous Topic: Overview of Computer Systems
- Relevant Slides:
- What is a Processor Programming Model? #card
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- A Processor Programming Model defines ^^how instructions access their operands and how instructions are described in the processor's assembly language.^^
- Processors with different programming models can offer similar sets of operations but may require very different approaches to programming.
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Instructions
- What is the Instruction Cycle? #card
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- The Instruction Cycle is the ^^procedure of processing an instruction^^ by the microprocessor.
- Fetch: read the instructions from memory
- Decode: Determine what is to be done
- Execute: Perform the operation
- Each of the functions fetch -> decode -> execute consist of a sequence of one or more operations inside the CPU (and interaction with the subsystems).
- The Instruction Cycle is the ^^procedure of processing an instruction^^ by the microprocessor.
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Types of Instructions
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Data Transfer Instructions
- What are Data Transfer Instructions? #card
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- Operations that ^^move data^^ from one place to another.
- These instructions ^^don't modify^^ the data, they just copy it to the destination.
- What operations can data transfer instructions do? #card
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- Load data from memory into the microprocessor. #card
- These instructions copy data from memory into microprocessor registers (i.e., LD).
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2. Store data from the microprocessor into the memory. #card
- Similar to load data, except that the data is copied in the opposite direction (i.e., ST).
- Data is saved from internal microprocessor registers into the memory
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3. Move data within the microprocessor.
- These instructions move data from one microprocessor register to another (i.e., MOV)
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4. Input data into the microprocessor.
- A microprocessor may need to input data from the outside world.
- These are the instructions that input data from the input device into the microprocessor.
- A microprocessor may need to input data from the outside world.
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5. Output data from the microprocessor.
- The microprocessor copies data from one of its internal registers to an output device.
- Example: the microprocessor may want to show the content of an internal register on a display (the key has been pressed) (i.e., IOWR).
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- What are Data Transfer Instructions? #card
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Data Operation Instructions #card
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- They typically perform some operation (e.g., +, -, *) using one or two data values (operands) and store the result.
- What operations can data operation instructions do? #card
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- Arithmetic Instructions #card
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- add, subtract, multiply, or divide
- ADD, SUB, MUL, DIV
- Instructions that increment or decrement one from a value
- INC, DEC
- Floating point instructions that operate on floating point values
- FADD, FSUB, FMUL, FDIV
- add, subtract, multiply, or divide
- Logic Instructions
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- AND, OR, XOR, NOT, etc.
- Shift Instructions
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- SR, SL, RR, RL, etc.
- Arithmetic Instructions #card
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Program Control Instructions #card
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- e.g., if, then, else.
- Instructions that can generate interrupts.
- Software interrupts.
- Jump & branch instructions (conditional or unconditional)
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- JZ: Jump if the zero flag is set.
- JNZ: Jump if the zero flag is not set.
- JMP: Unconditional jump - flags are ignored.
- etc.
- Comparison Instructions #card
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- TEST: logical BITWISE AND
- Calls & Returns a / from a routine (conditional or unconditional) #card
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- Call: call a subroutine at a certain line.
- RET: return from a subroutine.
- IRET: interrupt & return.
- Software Interrupts #card
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- Generated by devices outside of a microprocessor (not part of the instruction set).
- INT
- Exceptions & Traps #card
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- Triggered when valid instructions perform invalid operations.
- e.g., dividing by zero.
- Triggered when valid instructions perform invalid operations.
- Halt Instructions #card
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- Causes the processor to stop executions.
- e.g., at the end of the program.
- HALT
- Causes the processor to stop executions.
- Jump or branch instructions are used to ^^go to another part of the program^^; Jumps can be absolute or conditional.
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- What is the Instruction Cycle? #card
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Stack Architectures
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The Stack
- Last In First Out (LIFO) data structure.
- Consists of locations, each of which can hold a word of data.
- It can be used to explicitly save / restore data.
- What operations does the stack support? #card
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- The stack supports ^^two operations.^^
- PUSH: takes one argument and places the value of the argument at the top of the stack.
- POP: removes one element from the stack, saving it into a predefined register of the processor.
- The stack supports ^^two operations.^^
- The stack is ^^used implicitly by procedure call instructions.^^
- (if available in the data set).
- When new data is added to the stack, it is placed at the top of the stack, and all of the contents of the stack are pushed down one location.
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Implementing Stacks
- What are the two ways to implement a stack? #card
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- Dedicated Hardware Stack #card
- Has a ^^hardware limitation^^ (limited number of locations).
- Very fast.
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2. Memory Implemented Stack #card
- Limited by the ^^physical memory of the system.^^
- Slow compared with hardware stacks, since extra memory addressing has to take place for each stack operation.
{:height 405, :width 638}- Every push operation will ^^increment the top of the stack pointer^^ with the word size of the machine.
- Every pop operation will ^^decrement the top of the stack pointer^^ (with the word size of the machine).
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- Stack overflows can occur in both stack implementations
- What is a stack overflow? #card
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- A stack overflow occurs when the amount of data in the stack exceeds the amount of space allocated to the stack (or the hardware limit of the stack).
- What is a stack overflow? #card
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- What are the two ways to implement a stack? #card
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Instructions in Stack-Based Architecture #card
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- The advantage of this is that ^^program code takes little memory - there is no need to specify the address of the operands or registers.^^
- PUSH is one exception, because it needs the operand to be specified (either as a constant or as an address).
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Programs in a Stack-Based Architecture
- Writing programs for stack-based architecture is not easy.
- Stack-based architectures are better suited for postfix notation rather than infix notation.
- What is infix notation? #card
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- Infix notation is the traditional way of representing mathematical expressions, with ^^operations placed between the operands.^^
- e.g., a + b
- Infix notation is the traditional way of representing mathematical expressions, with ^^operations placed between the operands.^^
- What is postfix notation? #card
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- In postfix notation, ^^the operation is placed after the operands.^^
- e.g., a b +
- Stack-based architectures are better suited for postfix notation.
- Once an expression has been converted into postfix notation, implementing it in programs is easy.
- In postfix notation, ^^the operation is placed after the operands.^^
- Writing programs for stack-based architecture is not easy.
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Using Stacks to Implement Procedure Calls
- Programs need a way to pass inputs to the procedures that they call and to receive outputs back from them.
- Procedures need to be able to allocate space in memory for local variables without overriding any data used by their calling program.
- It is impossible to determine which registers may be used safely by the procedure (especially if the procedure is located in a library).
- So, a mechanism to save / restore registers of the calling program has to be in place.
- Procedures need a way to figure out where they were called from.
- So, the execution can return to the calling program where the procedure completes (they need to restore the program counter).
- How are procedure calls implemented in Stacks? #card
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- When a procedure is called,^^a block of memory in the stack called a stack frame is allocated.^^
- The top of the stack pointer is incremented by the number of locations in the stack frame.
- When a procedure finishes, it jumps to the return address of the stack and the execution of the calling program resumes.
- How are nested procedure calls implemented in the stack? #card
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- main program calls function f(),
- function f() calls function g(),
- function g() calls function h()
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General-Purpose Register Architectures
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General-Purpose Register File
- In GPR Architectures, instructions read their operands and write their results to a random access register file.
- The general-purpose register file allows the ^^access of any register in any order^^ by specifying the number (register ID) of the register.
- What is the main difference between a GPR & a stack? #card
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- The main difference between a GPR and a stack is that repeatedly reading a register will produce the same result and will not modify the state of the register file.
- Popping an item from a LIFO structure (stack) will modify the contents of the stack,
- The main difference between a GPR and a stack is that repeatedly reading a register will produce the same result and will not modify the state of the register file.
- Many GPR architectures assign special values to some registers in the register file to make programming easier.
- e.g., sometimes, register 0 is hardwired with value 0 to generate this most common constant.
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Instructions in GPR Architecture
- What do GPR instructions need to specify? #card
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- GPR instructions need to specify:
- the register that holds their input operands
- the register that will hole the result
- GPR instructions need to specify:
- What is the most common GPR instruction format? #card
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- The most common GPR instruction format is the three operands instruction format.
- e.g., "ADD r1, r2, r3" instructs the processor to read the contents of r2 and r3, add them together, and write the results in r1.
- Instructions that only have one or two inputs are also present in GPR architecture.
- The most common GPR instruction format is the three operands instruction format.
- Which architecture allows caching? #card
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- In GPR Architecture, ^^programs can choose which values should be stored in the register file at any given time^^, allowing them to cache the most accessed data.
- In stack-based architectures, once the data has been used, it's gone.
- From this point of view, ^^GPR architectures have better performance^^, at the expense of needing more storage space for the program.
- larger instructions are needed to encode the addresses of the operands.
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Simple GPR Instruction Set
- What do GPR instructions need to specify? #card
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Programs in a GPR Architecure #card
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- There are fewer restrictions on the order in which operations can be executed.
- In stack-based architectures, instructions must execute in the order that would leave the operands for the next instructions on the top of the stack.
- In GPR, any order that places the operands for the next instruction in the register file before the instruction executes is valid.
- Operations that access different registers can be reordered without making the program invalid.
- There are fewer restrictions on the order in which operations can be executed.
- Programming a GPR architecture processor is less structured than programming a stack-based architecture processor.
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Stack-Bases vs GPR Architectures
- Stack-based architectures are still attractive for certain embedded systems.
- GPR architectures are used by modern computers.
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Stack-Based Architectures #card
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- Reduced amount of memory taken up by programs.
- Manages the use of registers automatically (no need for programmer intervention).
- The instruction set does not change if the size of the register file has changed.
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GPR Architectures
- With the evolution of technology, the amount of space taken up by a program is less important.
- Compilers for GPR architecture achieve better performance with a given number of general-purpose registers than those on stack-based architectures with the same number of registers.
- The compiler can choose which values to keep (cache) in the register file at any time.
