Arm A-profile architecture: what does ZA stand for?

Question

This is a simple question, however, in Arm Architecture Reference Manual for A-profile architecture (issue I.a) I cannot find an answer.

What does ZA stand for?

Some examples (from the document above):

• ZA mode
• ZA tile
• ZA tile storage
• ZA array

Some resources mentioning ZA:

• https://arm-software.github.io/acle/main/acle.html#introduction-to-za-storage\
• https://reviews.llvm.org/D105570

Answer 1

ZA is the name of a large 2-dimensional array register that is part of the Armv9-A Scalable Matrix Extension (SME). It is described in Chapter B2 of that document (I'm using version A.a). Arm's docs often refer to it as "storage" instead of as a register, probably due to its size of at least 256 bytes (the square of the scalable vector length).

The A in ZA presumably stands for "array". I don't think the Z stands for anything but matches the Z0-Z31 naming for SVE registers.

Specifically:

• "ZA mode" refers to the state when "ZA storage is enabled", which can be selected with appropriate MSR instructions (see B1.1.1.3). The ZA register is only accessible in this mode.

• A "ZA tile" is a square sub-matrix of ZA. See B2.2.2.