A microassembler (sometimes called a meta-assembler) is a computer program that helps prepare a microprogram to control the low level operation of a computer in much the same way an assembler helps prepare higher level code for a processor. The difference is that the microprogram is usually only developed by the processor manufacturer and works intimately with the hardware. The microprogram defines the instruction set any normal program (including both application programs and operating systems) is written in. The use of a microprogram allows the manufacturer to fix certain mistakes, including working around hardware design errors, without modifying the hardware. Another means of employing microassembler-generated microprograms is in allowing the same hardware to run different instruction sets. After it is assembled, the microprogram is then loaded to a control store to become part of the logic of a CPU's control unit.
Some microassemblers are more generalized and are not targeted at a single computer architecture. 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 customized microcode.
In the process of microcode assembly it is helpful to verify the microprogram with emulating tools before distribution. Nowadays, microcoding experience a revival, since it is possible to correct and optimize the firmware (i.e. the microcode) of processing units sold, in order for adaptation to operating systems or for bug fixing means. However, a commonly usable microassembler for today's CPUs is not available to manipulate the microcode. Unfortunately, it is difficult to obtain open knowledge about changing the microcode because of intellectual property reasons.
How microcode can be assembled with a microassembler to control a CPU with own created machine codes on microprogramming basis, can be understood and simulated with e-learning tools like Mikrocodesimulator MikroSim on a didactial point of view.
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