(Some of this is gleaned from an article in the September 1986
Hewlett-Packard Journal and info in the July 1987 HP Communicator)
Page and segment table entries mimic the Motorola 68851 PMMU,
in an effort at upward compatibility. The HP MMU uses a two
level translation scheme. There are separate (but equal!)
translation tables for both supervisor and user modes. At the
lowest level are page tables. Each page table consists of one
or more 4k pages of 1024x4 byte page table entries. Each PTE
maps one 4k page of VA space. At the highest level is the
segment table. The segment table is a single 4K page of 1024x4
byte entries. Each entry points to a 4k page of PTEs. Hence
one STE maps 4Mb of VA space and one page of STEs is sufficient
to map the entire 4Gb address space (what a coincidence!). The
unused valid bit in page and segment table entries must be
zero.
There are separate translation lookaside buffers for the user
and supervisor modes, each containing 1024 entries.
To augment the 68020's instruction cache, the HP CPU has an
external cache. A direct-mapped, virtual cache implementation
is used with 16 Kbytes of cache on 320 systems and 32 Kbytes on
350 systems. Each cache entry can contain instructions or data,
from either user or supervisor space. Separate valid bits are
kept for user and supervisor entries, allowing for discriminatory
flushing of the cache.
MMU translation and cache-miss detection are done in parallel.
Segment table entries:
------- ----- -------
bits 31-12: Physical page frame number of PT page
bits 11-4: Reserved at zero
(can software use them?)
bit 3: Reserved at one
bit 2: Set to 1 if segment is read-only, ow read-write
bits 1-0: Valid bits
(hardware uses bit 1)
Page table entries:
---- ----- -------
bits 31-12: Physical page frame number of page
bits 11-7: Available for software use
bit 6: If 1, inhibits caching of data in this page.
(both instruction and external cache)
bit 5: Reserved at zero
bit 4: Hardware modify bit
bit 3: Hardware reference bit
bit 2: Set to 1 if page is read-only, ow read-write
bits 1-0: Valid bits
(hardware uses bit 0)
Hardware registers:
-------- ---------
The hardware has four longword registers controlling the MMU.
The registers can be accessed as shortwords also (remember to
add 2 to addresses given below).
5F4000: Supervisor mode segment table pointer. Loaded (as longword)
with page frame number (i.e. Physaddr >> 12) of the segment
table mapping supervisor space.
5F4004: User mode segment table pointer. Loaded (as longword) with
page frame number of the segment table mapping user space.
5F4008: TLB control register. Used to invalid large sections of the
TLB. More info below.
5F400C: MMU command/status register. Defined as follows:
bit 15: If 1, indicates a page table fault occurred
bit 14: If 1, indicates a page fault occurred
bit 13: If 1, indicates a protection fault (write to RO page)
bit 6: MC68881 enable. Tied to chip enable line.
(set this bit to enable)
bit 5: MC68020 instruction cache enable. Tied to Instruction
cache disable line. (set this bit to enable)
bit 3: If 1, indicates an MMU related bus error occurred.
Bits 13-15 are now valid.
bit 2: External cache enable. (set this bit to enable)
bit 1: Supervisor mapping enable. Enables translation of
supervisor space VAs.
bit 0: User mapping enable. Enables translation of user
space VAs.
Any bits set by the hardware are cleared only by software.
(i.e. bits 3,13,14,15)
Invalidating TLB:
------------ ---
All translations:
Read the TLB control register (5F4008) as a longword.
User translations only:
Write a longword 0 to TLB register or set the user
segment table pointer.
Supervisor translations only:
Write a longword 0x8000 to TLB register or set the
supervisor segment table pointer.
A particular VA translation:
Set destination function code to 3 ("purge" space),
write a longword 0 to the VA whose translation we are to
invalidate, and restore function code. This apparently
invalidates any translation for that VA in both the user
and supervisor LB. Here is what I did:
#define FC_PURGE 3
#define FC_USERD 1
_TBIS:
movl sp@(4),a0 | VA to invalidate
moveq #FC_PURGE,d0 | change address space
movc d0,dfc | for destination
moveq #0,d0 | zero to invalidate?
movsl d0,a0@ | hit it
moveq #FC_USERD,d0 | back to old
movc d0,dfc | address space
rts | done
Invalidating the external cache:
------------ --- -------- -----
Everything:
Toggle the cache enable bit (bit 2) in the MMU control
register (5F400C). Can be done by ANDing and ORing the
register location.
User:
Change the user segment table pointer register (5F4004),
i.e. read the current value and write it back.
Supervisor:
Change the supervisor segment table pointer register
(5F4000), i.e. read the current value and write it back.
