Chapter 10
Instruction Sets:
Characteristics
and Functions

What is an instruction set? - The complete collection of instructions that are understood by a CPU - is the part of the computer architecture related to programming, including the native data types, instructions, registers,addressing modes, memory architecture, interrupt and exception handling, and external I/O. An ISA includes a specification of the set of opcodes (machine language), and the native commands implemented by a particular processor. - Machine Code - Binary - Usually represented by assembly codes

Elements of an Instruction -Operation code (Op code) - Do this - Specifies the operation to be performed (e.g.. ADD, I/O). The operation is specified by a binary code, known as the operation code, or opcode. - Source Operand reference - To this - The operation may involve one or more source operands, that is, operands that are inputs for the operation. - Result Operand reference - Put the answer here - The operation may produce a result. - Next Instruction Reference - When you have done that, do this... - This tells the CPU where to fetch the next instruction after the execution of this instruction is complete.

Where have all the Operands gone? - The next instruction to be fetched is located in main memory or, in the case of a virtual memory system, in either main memory or secondary memory (disk). In most cases, the next instruction to be fetched immediately follows the current instruction. In those cases, there is no explicit reference to the next instruction. Source and result operands can be in one of three areas: - Main Memory (or virtual memory or cache) - As with next instruction references, the main or virtual memory address must be supplied. - CPU register - With rare exceptions, a CPU contains one or more registers that may be referenced by machine instructions. If only one register exists, reference to it may be implicit. If more than one register exists, then each register is assigned a unique number, and the instruction must contain the number of the desired register. - I/O device - The instruction must specify the I/O module and device for the operation. If memory-mapped I/O is used, this is just another main or virtual memory address.

Instruction Representation - In machine code each instruction has a unique bit pattern - For human consumption (well, programmers anyway) a symbolic representation is used - e.g. ADD, SUB, LOAD - Operands can also be represented in this way - ADD A,B

Instruction Types - Data processing - Data storage (main memory) - Data movement (I/O) - Program flow control

Number of Addresses (a)
3 addresses - Operand 1, Operand 2, Result - a = b + c; - May be a forth - next instruction (usually implicit) - Not common - Needs very long words to hold everything

Number of Addresses (b)
2 addresses - One address doubles as operand and result - a = a + b - Reduces length of instruction - Requires some extra work - Temporary storage to hold some results

Number of Addresses (c)
1 address - Implicit second address - Usually a register (accumulator) - Common on early machines

Number of Addresses (d)
0 (zero) addresses - All addresses implicit - Uses a stack - e.g. push a - push b - add - pop c

- c = a + b

How Many Addresses - More addresses - More complex (powerful?) instructions - More registers - Inter-register operations are quicker - Fewer instructions per program - Fewer addresses - Less complex (powerful?) instructions - More instructions per program - Faster fetch/execution of instructions

Design Decisions (1) - Operation repertoire - How many ops? - What can they do? - How complex are they? - Data types - Instruction formats - Length of op code field - Number of addresses

Design Decisions (2) - Registers - Number of CPU registers available - Which operations can be performed on which registers? - Addressing modes (later…) - RISC v CISC

Types of Operand - Addresses - Numbers - Integer/floating point - Characters - ASCII etc. - Logical Data - Bits or flags

Specific Data Types - General - arbitrary binary contents - Integer - single binary value - Ordinal - unsigned integer - Unpacked BCD - One digit per byte - Packed BCD - 2 BCD digits per byte - Near Pointer - 32 bit offset within segment - Bit field - Byte String - Floating Point

Types of Operation - Data Transfer - Arithmetic - Logical - Conversion - I/O - System Control - Transfer of Control

Data Transfer - Specify - Source - Destination - Amount of data - May be different instructions for different movements - e.g. IBM 370 - Or one instruction and different addresses - e.g. VAX

Arithmetic - Add, Subtract, Multiply, Divide - Signed Integer - Floating point ? - May include - Increment (a++) - Decrement (a--) - Negate (-a)

Logical - Bitwise operations - AND, OR, NOT

Conversion - E.g. Binary to Decimal
Input/Output
- May be specific instructions - May be done using data movement instructions (memory mapped) - May be done by a separate controller (DMA)
Systems Control - Privileged instructions - CPU needs to be in specific state - Ring 0 on 80386+ - Kernel mode - For operating systems use
Transfer of Control - Branch - e.g. branch to x if result is zero - Skip - e.g. increment and skip if zero - ISZ Register1 - Branch xxxx - ADD A

- Subroutine call - c.f. interrupt call

Byte Order(A portion of chips?) - What order do we read numbers that occupy more than one byte - e.g. (numbers in hex to make it easy to read) - 12345678 can be stored in 4x8bit locations as follows

Byte Order (example)
Address Value (1) Value(2)
184 12 78
185 34 56
186 56 34
186 78 12

i.e. read top down or bottom up?

Byte Order Names - The problem is called Endian - The system on the left has the least significant byte in the lowest address - This is called big-endian - The system on the right has the least significant byte in the highest address - This is called little-endian