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Opcode Table
An opcode table (also called an opcode matrix) is a visual representation of all opcodes in an instruction set. It is arranged such that each axis of the table represents an upper or lower nibble, which combined form the full byte of the opcode. Additional opcode tables can exist for additional instructions created using an opcode prefix. Table values The structure and arrangement of an opcode table appears as follows: Each cell from 00-FF contains information about the operation such as the equivalent assembly instruction corresponding to the opcode, parameters, and CPU cycle counts. Example opcode table This is the opcode table for the MOS Technology 6502 microprocessor from 1975. The 6502 uses 8-bit opcodes. Of the 256 possible opcodes available using an 8-bit pattern, the original 6502 uses only 151 of them, organized into 56 instructions with (possibly) multiple addressing modes. Because not all the 256 opcodes are not used, some opcode spaces are blank and the low nibble c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Opcode
In computing, an opcode (abbreviated from operation code) is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs), central processing units (CPUs), and software instruction sets. In ALUs, the opcode is directly applied to circuitry via an input signal bus. In contrast, in CPUs, the opcode is the portion of a machine language instruction that specifies the operation to be performed. CPUs Opcodes are found in the machine language instructions of CPUs as well as in some abstract computing machines. In CPUs, an opcode may be referred to as an instruction machine code, instruction code, instruction syllable, instruction parcel, or opstring. For any particular processor (which may be a general CPU or a more specialized processing unit), the opcodes are defined by the processor's instruction set architecture (ISA). They can be described using an opcode table. The types of operations may in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Nibble
In computing, a nibble, or spelled nybble to match byte, is a unit of information that is an aggregation of four- bits; half of a byte/ octet. The unit is alternatively called nyble, nybl, half-byte or tetrade. In networking or telecommunications, the unit is often called a semi-octet, quadbit, or quartet. As a nibble can represent sixteen () possible values, a nibble value is often shown as a hexadecimal digit (hex digit). A byte is two nibbles, and therefore, a value can be shown as two hex digits. Four-bit computers use nibble-sized data for storage and operations; as the word unit. Such computers were used in early microprocessors, pocket calculators and pocket computers. They continue to be used in some microcontrollers. In this context, 4-bit groups were sometimes also called characters rather than nibbles. History The term ''nibble'' originates from its representing half a byte, with ''byte'' a homophone of the English word ''bite''. In 2014, David B. Be ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Opcode Prefix
In computing, an opcode prefix is an numeric value that alters the function of a following opcode. On some instruction set architectures multiple opcode prefixes are allowed sequentially, with all combining to alter the subsequent opcode. The opcode prefix is a portion of a larger machine language instruction that specifies the operation to be performed. In addition to the opcode, some instructions specify the operands the operation will act upon. Opcode prefixes may alter the number or size of the operands. RISC processors do not use opcode prefixes. Types of prefixes Opcode prefixes generally fall into two categories: # Opcode prefixes that alter subsequent opcode functions. # Opcode prefixes that enable an alternate set of opcodes. This is analogous to the shift key on a keyboard. In this way, instruction sets can be extended by adding a subset of new instructions by reusing existing reserved opcodes. Example opcode prefixes The following is not intended to be an exhaustiv ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
MOS Technology 6502
The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit computing, 8-bit microprocessor that was designed by a small team led by Chuck Peddle for MOS Technology. The design team had formerly worked at Motorola on the Motorola 6800 project; the 6502 is essentially a simplified, less expensive and faster version of that design. When it was introduced in 1975, the 6502 was the least expensive microprocessor on the market by a considerable margin. It initially sold for less than one-sixth the cost of competing designs from larger companies, such as the 6800 or Intel 8080. Its introduction caused rapid decreases in pricing across the entire processor market. Along with the Zilog Z80, it sparked a series of projects that resulted in the home computer microcomputer revolution, revolution of the early 1980s. Home video game consoles and home com ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Addressing Mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how the machine language instructions in that architecture identify the operand(s) of each instruction. An addressing mode specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere. In computer programming, addressing modes are primarily of interest to those who write in assembly languages and to compiler writers. For a related concept see orthogonal instruction set which deals with the ability of any instruction to use any addressing mode. Caveats There are no generally accepted names for addressing modes: different authors and computer manufacturers may give different names to the same addressing mode, or the same names to different addressing modes. F ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
