Entry point mapping and skipping method and apparatus

DETAILED DESCRIPTION The present invention uses a fixed entrypoint map to produce an entry point address of a first micro-instruction for a particular macro-instruction.
That address is then incremented by a fixed number, such as 512, to produce the second, third, etc.
microinstructions for that macro-instruction.
After a fixed number of these 512 address skips, such as 6, the addresses are incremented by 1 so that successive micro-instructions are in adjacent address locations.
The determination of whether to increment the address by 512 or by 1 is done by a decode circuit which simply looks at the current address to determine whether it is in the skipping region of the microcode memory or the successive incrementing region.
The present invention thus provides flexibility to the micro-instruction addressing with the minimal addition of circuitry.
In addition, the method of the present invention allows the entry point map to be fixed, and thus there is no need for it to be programmable.
The macro-instructions are arranged so that any macro-instruction requiring more than six micro-instructions is followed by one or more macro-instructions which require six or less micro-instructions.
Thus, when the first macro-instruction starts executing the micro-instructions in the increment by one region of memory, these memory addresses will be available since the next succeeding macroinstructions will not extend that far.
Alternately, a variable skipping region can be used to give more flexibility.
This flexibility enables the skipping regions to be determined when the code is written, thus enabling the optimization of memory space.
A variable skipping region implementation requires a slightly more complex method of determining whether the address should be an increment of one or a skip increment upon returns from subroutine calls, since it is not simply dependent upon the fixed address.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings