Memory Integrity Enforcement

1988 called and wants it memory tagging back https://www.devever.net/~hl/ppcas !

But yeah this was support for a the longest time by IBM basically. It's nice to see it's getting more widespread.

The big difference with this seems like it is an actual security mechanism to block "invalid" accesses where as the tagged memory extensions only provided pointer metadata and it was up to the OS to enforce invariants.

> Extensions provide no security. [...] The tagged memory extensions don't stop you from doing anything.

The problem with PowerPC AS tagging was that it relied entirely on the trap instruction. If you could control execution at all, you could skip the trap instruction and it did nothing. This implementation, by my reading, essentially adds a synchronous trap instruction after every single load and store, which builds a real security boundary (even compared to Android MTE, where reads would trap but writes were only checked at the next context switch).

Nitpick: The AS/400 in 1988 didn't use the PowerPC. I believe it had it's own proprietary memory with tag bits included.

The first RS-64 with the PowerPC AS extensions came out in 1995.

SPARC ADI was a predecessor to ARM MTE. ARM MTE has been available and used in production for several years now. ADI is also 4 bit but with 64 byte granularity rather than 16 byte.