External links
- i8051 SFRs
- SPECIAL FUNCTION REGISTERS 1/2 at the Wayback Machine (archived 2014-01-29)
- SFRs in C programming for AVR
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A Special Function Register (or Special Purpose Register, or simply Special Register) is a register within a microprocessor, which controls or monitors various aspects of the microprocessor's function. Depending on the processor architecture, this can include, but is not limited to:
Because special registers are closely tied to some special function or status of the processor, they might not be directly writeable by normal instructions (such as adds, moves, etc.). Instead, some special registers in some processor architectures require special instructions to modify them. For example, the program counter is not directly writeable in many processor architectures. Instead, the programmer uses instructions such as return from subroutine, jump, or branch to modify the program counter. For another example, the condition code register might not directly writable, instead being updated only by compare instructions.
Some of SFR (Special Function Register) bits may be set directly using SETB/LDB instructions on proper address, whereas others may require usage of specific instructions. The Intel 80196 class microcontroller has 24 SFRs, each 1 Byte in size; standard Intel 8051 chips have 21 SFRs.
The Special Function Register (SFR) is the upper area of addressable memory, from address 0x80 to 0xFF. This area of memory can't be used for data or program storage, but is instead a series of memory-mapped ports and registers. All port input and output can therefore be performed by memory move operations on specified addresses in the SFR. Also, different status registers are mapped into the SFR, for use in checking the status of the 8051, and changing some operational parameters of the 8051.
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The Intel 8080 ("eighty-eighty") is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier 8008 design, although without binary compatibility. The initial specified clock rate or frequency limit was 2 MHz, with common instructions using 4, 5, 7, 10, or 11 cycles. As a result, the processor is able to execute several hundred thousand instructions per second. Two faster variants, the 8080A-1 and 8080A-2, became available later with clock frequency limits of 3.125 MHz and 2.63 MHz respectively. The 8080 needs two support chips to function in most applications: the i8224 clock generator/driver and the i8228 bus controller. It is implemented in N-type metal–oxide–semiconductor logic (NMOS) using non-saturated enhancement mode transistors as loads thus demanding a +12 V and a −5 V voltage in addition to the main transistor–transistor logic (TTL) compatible +5 V.
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus, and is notable as the processor used in the original IBM PC design.
In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an implementation.
The program counter (PC), commonly called the instruction pointer (IP) in Intel x86 and Itanium microprocessors, and sometimes called the instruction address register (IAR), the instruction counter, or just part of the instruction sequencer, is a processor register that indicates where a computer is in its program sequence.
The Intel MCS-51 is a single chip microcontroller (MCU) series developed by Intel in 1980 for use in embedded systems. The architect of the Intel MCS-51 instruction set was John H. Wharton. Intel's original versions were popular in the 1980s and early 1990s, and enhanced binary compatible derivatives remain popular today. It is a complex instruction set computer, but also has some of the features of RISC architectures, such as a large register set and register windows, and has separate memory spaces for program instructions and data.
The Motorola 68000 series is a family of 32-bit complex instruction set computer (CISC) microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and were the primary competitors of Intel's x86 microprocessors. They were best known as the processors used in the early Apple Macintosh, the Sharp X68000, the Commodore Amiga, the Sinclair QL, the Atari ST, the Sega Genesis, the Capcom System I (Arcade), the AT&T UNIX PC, the Tandy Model 16/16B/6000, the Sun Microsystems Sun-1, Sun-2 and Sun-3, the NeXT Computer, NeXTcube, NeXTstation, and NeXTcube Turbo, the Texas Instruments TI-89/TI-92 calculators, the Palm Pilot, the Control Data Corporation CDCNET Device Interface, and the Space Shuttle. Although no modern desktop computers are based on processors in the 680x0 series, derivative processors are still widely used in embedded systems.
The Intel 8008 is an early byte-oriented microprocessor designed by Computer Terminal Corporation (CTC), implemented and manufactured by Intel, and introduced in April 1972. It is an 8-bit CPU with an external 14-bit address bus that could address 16 KB of memory. Originally known as the 1201, the chip was commissioned by Computer Terminal Corporation (CTC) to implement an instruction set of their design for their Datapoint 2200 programmable terminal. As the chip was delayed and did not meet CTC's performance goals, the 2200 ended up using CTC's own TTL-based CPU instead. An agreement permitted Intel to market the chip to other customers after Seiko expressed an interest in using it for a calculator.
The Intel 8085 ("eighty-eighty-five") is an 8-bit microprocessor produced by Intel and introduced in March 1976. It is software-binary compatible with the more-famous Intel 8080 with only two minor instructions added to support its added interrupt and serial input/output features. However, it requires less support circuitry, allowing simpler and less expensive microcomputer systems to be built. The "5" in the part number highlighted the fact that the 8085 uses a single +5-volt (V) power supply by using depletion-mode transistors, rather than requiring the +5 V, −5 V and +12 V supplies needed by the 8080. This capability matched that of the competing Z80, a popular 8080-derived CPU introduced the year before. These processors could be used in computers running the CP/M operating system.
The Intel 4040 microprocessor was the successor to the Intel 4004. It was introduced in 1974. The 4040 employed a 10 μm silicon gate enhancement load PMOS technology, was made up of 3,000 transistors and could execute approximately 62,000 instructions per second. General performance, bus layout and instruction set was identical to the 4004, with the main improvements being in the addition of extra lines and instructions to recognise and service interrupts and hardware Halt/Stop commands, an extended internal stack and general-purpose "Index" register space to handle nesting of several subroutines and/or interrupts, plus a doubling of program ROM address range.
x86 assembly language is the name for the family of assembly languages which provide some level of backward compatibility with CPUs back to the Intel 8008 microprocessor, which was launched in April 1972. It is used to produce object code for the x86 class of processors.
A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. In computer architecture, registers are typically addressed by mechanisms other than main memory, but may in some cases be assigned a memory address e.g. DEC PDP-10, ICT 1900.
A branch is an instruction in a computer program that can cause a computer to begin executing a different instruction sequence and thus deviate from its default behavior of executing instructions in order. Branch may also refer to the act of switching execution to a different instruction sequence as a result of executing a branch instruction. Branch instructions are used to implement control flow in program loops and conditionals.
The TMS9900 was one of the first commercially available, single-chip 16-bit microprocessors. Introduced in June 1976, it implemented Texas Instruments' TI-990 minicomputer architecture in a single-chip format, and was initially used for low-end models of that lineup.
The Fairchild F8 is an 8-bit microprocessor system from Fairchild Semiconductor, announced in 1974 and shipped in 1975. The original processor family included four main 40-pin integrated circuits (ICs); the 3850 CPU which was the arithmetic logic unit, the 3851 Program Storage Unit (PSU) which contained 1 KB of program ROM and handled instruction decoding, and the optional 3852 Dynamic Memory Interface (DMI) or 3853 Static Memory Interface (SMI) to control additional RAM or ROM holding the user programs or data. The 3854 DMA was another optional system that added direct memory access into the RAM controlled by the 3852.
The Clipper architecture is a 32-bit RISC-like instruction set architecture designed by Fairchild Semiconductor. The architecture never enjoyed much market success, and the only computer manufacturers to create major product lines using Clipper processors were Intergraph and High Level Hardware, although Opus Systems offered a product based on the Clipper as part of its Personal Mainframe range. The first processors using the Clipper architecture were designed and sold by Fairchild, but the division responsible for them was subsequently sold to Intergraph in 1987; Intergraph continued work on Clipper processors for use in its own systems.
The Signetics 2650 was an 8-bit microprocessor introduced in July 1975. According to Adam Osborne's book An Introduction to Microprocessors Vol 2: Some Real Products, it was "the most minicomputer-like" of the microprocessors available at the time. A combination of missing features and odd memory access limited its appeal, and the system saw little use in the market.
In computer architecture, a transport triggered architecture (TTA) is a kind of processor design in which programs directly control the internal transport buses of a processor. Computation happens as a side effect of data transports: writing data into a triggering port of a functional unit triggers the functional unit to start a computation. This is similar to what happens in a systolic array. Due to its modular structure, TTA is an ideal processor template for application-specific instruction set processors (ASIP) with customized datapath but without the inflexibility and design cost of fixed function hardware accelerators.
The ST6 and ST7 are 8-bit microcontroller product lines from STMicroelectronics. They are commonly used in small embedded applications like washing machines.
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
The COP400 or COP II is a 4-bit microcontroller family introduced in 1977 by National Semiconductor as a follow-on product to their original PMOS COP microcontroller. COP400 family members are complete microcomputers containing internal timing, logic, ROM, RAM, and I/O necessary to implement dedicated controllers. Some COP400 devices were second-sourced by Western Digital as the WD4200 family. In the Soviet Union several COP400 microcontrollers were manufactured as the 1820 series.