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Among early examples of microcode in micros was the Intel 8086. [6] Among the ultimate implementations of microcode in microprocessors is the Motorola 68000. This offered a highly orthogonal instruction set with a wide variety of addressing modes, all implemented in microcode.
Intel microcode is microcode that runs inside x86 processors made by Intel. Since the P6 microarchitecture introduced in the mid-1990s, the microcode programs can be patched by the operating system or BIOS firmware to work around bugs found in the CPU after release. [ 1 ]
Special purpose instructions also often lack explicit operands; for example, CPUID in the x86 architecture writes values into four implicit destination registers. This distinction between explicit and implicit operands is important in code generators, especially in the register allocation and live range tracking parts.
Other designs employ microcode routines or tables (or both) to do this, using ROMs or writable RAMs (writable control store), PLAs, or both. Some microcoded CPU designs with a writable control store use it to allow the instruction set to be changed (for example, the Rekursiv processor and the Imsys Cjip). [19]
Microcode is a very commonly used technique "that imposes an interpreter between the hardware and the architectural level of a computer". [23] As such, the microcode is a layer of hardware-level instructions that implement higher-level machine code instructions or internal state machine sequencing in many digital processing elements.
Recent examples of similar open-sourced microsequencer-based processors are the MicroCore Labs MCL86, MCL51, and MCL65 cores which emulate the Intel 8086/8088, 8051 and MOS 6502 instruction sets entirely in microcode.
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For example, through the use of macro-assembler-like capabilities, Digital Equipment Corporation used their MICRO2 microassembler for a very wide range of computer architectures and implementations. If a given computer implementation supports a writeable control store , the microassembler is usually provided to customers as a means of writing ...