Opcode stands for Operation Code. Opcodes operate on processor register, values in memory, values stored on the Stack (computing), I/O ports, the Computer bus, etc. They are used to perform arithmetic operations and move and change values. Operands are the things that opcodes operate on. Microprocessors perform operations using binary bits. A Bit is either on (1) or off (0). Eight bits is equal to a byte (on most modern processors), and two bytes is equal to a word.

Another term for opcodes is Byte code. Byte code is used especially to describe higher level constructs as is the case with the Java programming language s JVM (Java Virtual Machine). For instance, byte code 1A might be the iconst_2 instruction which pushes the number 2 on the stack. This is a slightly higher level of abstraction than opcodes, which might need to load the number 2 into a register, and then push the register s value on the stack.

As an example let s design a crude 4-bit microprocessor. *First: All registers, ALUs, counters, and addresses must have a data path of 4-bits. *Second: All of our instructions must fit in a 3-bit address. Below is a diagram of the opcodes of the micorprocessor: op-codes mnemonic operations explanation 000 ADD add A to B and store in b 001 mov move A to B and store in b 010 Jmp jump value in A 011 xorA xor A with next op-code store in b 100 clrA clear A 101 return return to pointer 110 counter counter value 111 end end program

When the op-code values are active at the decoders logic inputs, the desired operations are performed. Each of these operations is assigned a numeric code, this is the opcode. To assist in the use of these numeric codes, mnemonics are used as textual abbreviations. It s much easier to remember ADD than 05, for example.

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