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Status Register
A status register, flag register, or condition code register is a collection of status flag bits for a processor. An example is the FLAGS register of the x86 architecture or flags in a program status word (PSW) register. The status register is a hardware register that contains information about the state of the processor. Individual bits are implicitly or explicitly read and/or written by the machine code instructions executing on the processor. The status register lets an instruction take action contingent on the outcome of a previous instruction. Typically, flags in the status register are modified as effects of arithmetic and bit manipulation operations. For example, a Z bit may be set if the result of the operation is zero and cleared if it is nonzero. Other classes of instructions may also modify the flags to indicate status
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AMD 29000
The AMD
AMD
Am29000 (commonly shortened to 29k) is a family of 32-bit
32-bit
RISC microprocessors and microcontrollers developed and fabricated by Advanced Micro Devices
Advanced Micro Devices
(AMD). They were, for a time, the most popular RISC
RISC
chips on the market, widely used in laser printers from a variety of manufacturers. In late 1995 AMD
AMD
dropped development of the 29k because the design team was transferred to support the PC side of the business. What remained of AMD's embedded business was realigned towards the embedded 186 family of 80186 derivatives
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Negative Flag
In a computer processor the negative flag or sign flag is a single bit in a system status (flag) register used to indicate whether the result of the last mathematical operation resulted in a value in which the most significant bit was set
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Very Long Instruction Word
Very long instruction word (VLIW) refers to instruction set architectures designed to exploit instruction level parallelism (ILP). Whereas conventional central processing units (CPU, processor) mostly allow programs to specify instructions to execute in sequence only, a VLIW processor allows programs to explicitly specify instructions to execute at the same time, concurrently, in parallel. This design is intended to allow higher performance without the complexity inherent in some other designs.Contents1 Overview 2 Motivation 3 Design 4 History 5 Implementations 6 Backward compatibility 7 See also 8 References 9 External linksOverview[edit]This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed
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Hazard (computer Architecture)
In the domain of central processing unit (CPU) design, hazards are problems with the instruction pipeline in CPU microarchitectures when the next instruction cannot execute in the following clock cycle,[1] and can potentially lead to incorrect computation results
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Speculative Execution
Speculative execution is an optimization technique where a computer system performs some task that may not be needed. Work is done before it is known whether it is actually needed, so as to prevent a delay that would have to be incurred by doing the work after it is known that it is needed. If it turns out the work was not needed after all, most changes made by the work are reverted and the results are ignored. The objective is to provide more concurrency if extra resources are available
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Superscalar
A superscalar processor is a CPU that implements a form of parallelism called instruction-level parallelism within a single processor. In contrast to a scalar processor that can execute at most one single instruction per clock cycle, a superscalar processor can execute more than one instruction during a clock cycle by simultaneously dispatching multiple instructions to different execution units on the processor. It therefore allows for more throughput (the number of instructions that can be executed in a unit of time) than would otherwise be possible at a given clock rate
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Instruction Pipeline
In the fourth clock cycle (the green column), the earliest instruction is in MEM stage, and the latest instruction has not yet entered the pipeline. Instruction pipelining
Instruction pipelining
is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming instructions into a series of sequential steps (the eponymous "pipeline") performed by different processor units with different parts of instructions processed in parallel
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Protection Ring
In computer science, hierarchical protection domains,[1][2] often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behaviour (by providing computer security). This approach is diametrically opposite to that of capability-based security.[citation needed] Computer operating systems provide different levels of access to resources. A protection ring is one of two or more hierarchical levels or layers of privilege within the architecture of a computer system. This is generally hardware-enforced by some CPU architectures that provide different CPU modes at the hardware or microcode level. Rings are arranged in a hierarchy from most privileged (most trusted, usually numbered zero) to least privileged (least trusted, usually with the highest ring number)
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Interrupt Handler
In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a special block of code associated with a specific interrupt condition. Interrupt
Interrupt
handlers are initiated by hardware interrupts, software interrupt instructions, or software exceptions, and are used for implementing device drivers or transitions between protected modes of operation, such as system calls. The traditional form of interrupt handler is the hardware interrupt handler. Hardware interrupts arise from electrical conditions or low-level protocols implemented in digital logic, are usually dispatched via a hard-coded table of interrupt vectors, asynchronously to the normal execution stream (as interrupt masking levels permit), often using a separate stack, and automatically entering into a different execution context (privilege level) for the duration of the interrupt handler's execution
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Interrupt Flag
IF ( Interrupt
Interrupt
Flag) is a system flag bit in the x86 architecture's FLAGS register, which determines whether or not the CPU will handle maskable hardware interrupts.[1] The bit, which is bit 9 of the FLAGS register, may be set or cleared by programs with sufficient privileges, as usually determined by the Operating System. If the flag is set to 1, maskable hardware interrupts will be handled. If cleared (set to 0), such interrupts will be ignored. IF does not affect the handling of non-maskable interrupts or software interrupts generated by the INT instruction.Contents1 Setting and clearing 2 Privilege level2.1 Old DOS
DOS
programs3 CLI 4 STI 5 See also 6 References 7 External linksSetting and clearing[edit] The flag may be set or cleared using the CLI (Clear Interrupts), STI (Set Interrupts) and POPF (Pop Flags) instructions. CLI clears IF (sets to 0), while STI sets IF to 1
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Parity Flag
In computer processors the parity flag indicates if the number of set bits is odd or even in the binary representation of the result of the last operation. It is normally a single bit in a processor status register. For example, assume a machine where a set parity flag indicates even parity. If the result of the last operation were 26 (11010 in binary), the parity flag would be 0 since the number of set bits is odd. Similarly, if the result were 10 (1010 in binary) then the parity flag would be 1. x86 processors[edit] In x86 processors, the parity flag reflects the parity only of the least significant byte of the result, and is set if the number of set bits of ones is even (put another way, the parity bit is set if the sum of the bits is even)
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Binary-coded Decimal
In computing and electronic systems, binary-coded decimal (BCD) is a class of binary encodings of decimal numbers where each decimal digit is represented by a fixed number of bits, usually four or eight. Special
Special
bit patterns are sometimes used for a sign or for other indications (e.g., error or overflow). In byte-oriented systems (i.e. most modern computers), the term unpacked BCD[1] usually implies a full byte for each digit (often including a sign), whereas packed BCD typically encodes two decimal digits within a single byte by taking advantage of the fact that four bits are enough to represent the range 0 to 9. The precise 4-bit encoding may vary however, for technical reasons, see Excess-3 for instance
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Nibble
In computing, a nibble (occasionally nybble or nyble to match the spelling of byte) is a four-bit aggregation,[1][2] or half an octet. It is also known as half-byte[3] or tetrade.[4][5] In a networking or telecommunication context, the nibble is often called a semi-octet,[6] quadbit,[7] or quartet.[8][9] A nibble has sixteen (24) possible values. A nibble can be represented by a single hexadecimal digit and called a hex digit.[10] A full byte (octet) is represented by two hexadecimal digits; therefore, it is common to display a byte of information as two nibbles. Sometimes the set of all 256 byte values is represented as a 16×16 table, which gives easily readable hexadecimal codes for each value. Four-bit computer architectures use groups of four bits as their fundamental unit. Such architectures were used in early microprocessors, pocket calculators and pocket computers
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Bit
The bit (a portmanteau of binary digit)[1] is a basic unit of information used in computing and digital communications. A binary digit can have only one of two values, and may be physically represented with a two-state device. These state values are most commonly represented as either a 0or1. The two values of a binary digit can also be interpreted as logical values (true/false, yes/no), algebraic signs (+/−), activation states (on/off), or any other two-valued attribute. The correspondence between these values and the physical states of the underlying storage or device is a matter of convention, and different assignments may be used even within the same device or program
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Overflow Flag
In computer processors, the overflow flag (sometime called V flag) is usually a single bit in a system status register used to indicate when an arithmetic overflow has occurred in an operation, indicating that the signed two's-complement result would not fit in the number of bits used for the operation (the ALU width). Some architectures may be configured to automatically generate an exception on an operation resulting in overflow.An illustrative example is what happens if we add 127 and 127 using 8-bit registers. 127+127 is 254, but using 8-bit arithmetic the result would be 1111 1110 binary, which is -2 in two's complement, and thus negative. A negative result out of positive operands (or vice versa) is an overflow. The overflow flag would then be set so the program can be aware of the problem and mitigate this or signal an error
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