Apple Lisa - Segment Origin; Context Selection

Confidential
Lisa Hardware Reference Manual
THE SEGMENT ORIGIN
The Segment Origin Register (SORG) contains the high 12 bits of the
address added to the segment offset produced by the 68000 to get the
physical address. !he register can be read or written. Its address
is
where
SSSSSSSxlOxxxxxxxxxxlxxx (in Special I/O space)
SSSSSSS
x
selects the segment
ignored
Bits 0 to 11 of the register set the segment base address for segment
SSSSSSS. !he other 4 bits in the register are not used. For
example, a segment whose origin is $000200 has SORGll - SORGO set to
$001. !he address is in the currently selected context. The SEGl
and SEG2 bits determine which context is selected. An MMU setup
program should run in supervisor mode. '
CONTEXT SELECTION
!he context bits mentioned above permit fast context switching. The
MMU has four sets of MMU registers which can be assigned to four
different processes. Once these contexts have been loaded, the
context bits can be used to control which context is active.
Contexts 1, 2, and 3 are general purpose contexts, but context 0 is
intended for the use of the Operating System.' Whenever the 68000
does an access in supervisor mode, the context is automatically
switched to context O. When the context selection bits are actually
set, however, the fact that you are necessarily in supervisor mode is
ignored and the desired context is selected. !he TRAP instruction,
, which puts the processor in supervisor mode, can be used to generate
a call from a user process to the Operating System.
The
two bits SEGl and SEG2 control which context in the MMU is
selected. SEGl is set by a read or write at the address $OOEOOA (in
I/O space), and reset by a read or write at $00E008. SEG2 is set by
a read or write at $OOEOOE and reset at $OOEOOC. The context
selected is determined as follows:
SEG2 SEGl ·Context
o
o
1
1
o
1
o
1
o
1
2
3
Page 12
7-Jul-81