> the data bus is 128 bits wide: 64-bit for the data and 64-bit for data's type
That seems a bit wasteful if you're not using a lot of object types.
That seems a bit wasteful if you're not using a lot of object types.
It has allowed the AS/400 to have a single-level store, which means that «memory» and «disk» live in one conceptual address space.
A pointer can carry more than just an address – object identity, type, authority metadata – AS/400 uses tagged 16-byte pointers to stop arbitrary pointer fabrication, which supports isolation without relying on the usual per-process address-space model in the same way UNIX does.
Such «fat pointer» approach is conceptually close to modern capability systems (for example CHERI’s 128-bit capabilities), which exist for similar [safety] reasons.
[0] 128-bit pointers in the machine interface, not a 128-bit hardware virtual address space though.
The software written for this virtual architecture, on the other hand, has not changed and continues to run on modern IBM iSeries systems, even when it originates from 1989 – this is accomplished through static binary translation, or AOT in modern parlance, which recompiles the virtual ISA into the target ISA at startup.