Demindiro: Hardwired zero register

2025-03-19T20:34:00Z

Hardwired zero register

← Older revision		Revision as of 20:34, 19 March 2025
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	24-bit xxxxxxxx xxxxxxxx xxxxx011		24-bit xxxxxxxx xxxxxxxx xxxxx011
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			=== Hardwired zero register ===

			A substantial amount of instructions can be omitted by having a register which always reads 0.

	=== Implicit destination registers ===		=== Implicit destination registers ===

Demindiro: Draft

2025-03-18T19:59:37Z

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There are many different ways to encode instructions, all with their own tradeoffs.

== Considerations ==

=== Fixed-width or variable-length ===

Even among variable-length ISAs there is a broad spectrum in complexity. For example, x86 is notorious for being difficult to decode. In contrast, RISC-V is also variable-length but trivial to decode.

=== Instruction density ===

Large instructions can encode many things but have poor density, while smaller instructions have good density but may not be able to encode many things.

Of particular note is the amount of directly addressable registers: a larger amount of registers requires more bits to encode, leaving less to encode other things.

=== Instruction formats ===

To reduce decoding complexity it is wise to define a few fixed formats, which all instructions should be defined in terms of.

== Techniques ==

=== Variable-length encoding ===

To keep decoding variable-length instructions simple one can use a fixed prefix of a few bits.

For example, an ISA with 8, 16 and 24-bit instructions could be encoded as:
<source>
23 16 15 8 7 0
8-bit xxxxxxx0
16-bit xxxxxxxx xxxxxx01
24-bit xxxxxxxx xxxxxxxx xxxxx011
</source>

=== Implicit destination registers ===

Instead of having explicitly named registers, one can instead have a queue of registers and address source registers relative to the "instruction distance".

Aside from being useful for out-of-order processors, this also avoids the need to encode a destination, freeing bits for other purposes.

Instruction encoding - Revision history

Demindiro: Hardwired zero register

Demindiro: Draft