Zba Extension

Versions

1.0.0
State

ratified

Ratification date

2021-06

Synopsis

The Zba instructions can be used to accelerate the generation of addresses that index into arrays of basic types (halfword, word, doubleword) using both unsigned word-sized and XLEN-sized indices: a shifted index is added to a base address.

The shift and add instructions do a left shift of 1, 2, or 3 because these are commonly found in real-world code and because they can be implemented with a minimal amount of additional hardware beyond that of the simple adder. This avoids lengthening the critical path in implementations.

While the shift and add instructions are limited to a maximum left shift of 3, the slli instruction (from the base ISA) can be used to perform similar shifts for indexing into arrays of wider elements. The slli.uw — added in this extension — can be used when the index is to be interpreted as an unsigned word.

Instructions

The following instructions are defined by this extension:

add.uw

Add unsigned word

sh1add.uw

Shift unsigend word left by 1 and add

sh1add

Shift left by 1 and add

sh2add.uw

Shift unsigend word left by 2 and add

sh2add

Shift left by 2 and add

sh3add.uw

Shift unsigend word left by 3 and add

sh3add

Shift left by 3 and add

slli.uw

Shift left unsigned word (Immediate)