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Proposal for instruction set - now on Github
Author: Agner Date: 2016-08-31 11:08
Joe Duarte wrote:
For example, maybe some instructions are only allowed at the start of a process. After a certain milestone, some instruction codes switch to represent different instructions – instructions that cannot be used at the start of a process, only after the milestone.
The Opcode numbers are not so crowded that this would be worth the extra complexity, but some code spaces are reserved for application-specific or user-defined instructions.


I wonder if we could use a powers-of-two type, such that the literal binary value would represent an exponent of two, e.g. 10001101 would mean 2^141. Something like that would help with encoding some large numbers, though powers of two might not be the right form.
ForwardCom includes formats that code constants as a << b = a · 2b, but only for immediate constants, not for variables. It also supports half precision floating point constants - but not variables.


Relatedly, instead of hard-coding a single constant like π the way I was thinking several months ago, there could be a type that represented a set of constants – maybe four or five bits to encode 16 to 32 predesignated constants that were empirically determined to recur frequently enough in a wide range of applications to be worth encoding.
If we have an extra set of registers, these could be used for constants that are reused in a program.
 
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