[GSoC] BAT vs Pages on PowerPC [Please give a feedback]

[Chris Johns]

Hesham Moustafa wrote:
> Hi there,
>
> I am working on providing memory protection for many targets/BSPs that
> RTEMS support through a library called libmm. Previous MMU projects
> depended on "Pages" to provide memory protection. I was thinking of
> using BATs for extending libmm for PowerPC targets. The following are
> some characteristics for BAT (Block Address Translation) and Pages for
> PowerPC/mpc6xx.
>
> For BATs :
> - BAT sizes varies from 128KB up to 256 MB.
> - 8 pairs of register to describe blocks of memory: 4 for data blocks
> and 4 for instructions blocks.
> - It's faster than Page translation.
> - Simple.
>
> For Pages :
> - Page sizes begins from 4KB.
> - Hash function are used for Page tables search and insert.
> - Slower than BAT.
> - Complex Code and cost much time when dynamically change attributes.
>
> The current code for libmm/PowerPC is using Pages. Do you think it's
> suitable to implement libmm to use BAT also ?
>
> Waiting for your feed back.
>

How many BAT areas can you have ?

Chris