Apple Lisa - Segment Limit

Confidential Lisa Hardware Reference Manual
THE SEGMENT LIMIT
When the MMU receives an address, the address is first relocated and
an access to main memory is started at the addressed location. The
MMU then checks to see that the relocated address is actually within
the limits of the segment being accessed.
LIMIT CHECKING
Logical Address
<- - - 7 - - -) <- - - - - - - - 17 - - - - - - - -)
+------------,----+-----------------------------------+
segment no. 1
------+
1
1
offset
1
logical page no 1
(8 bits)
displacement
(9 bits)
+-------+
+-------------+-------------+
I
I CNTRL
SLIM7 - SLIMO
1 +------~I-------------+
1 01 I
1 • 1 I
1 .1 1
+--)51 1111 11001111 !
.1
.1
I
I
• I I
1271 1
++--- .. -+--
v
-_ .• _. -+ -
I
I
I
---+
-i------------+ v
limit 1->[+]
+---------+ 1
v
7-Jul-S1
limit failure if carry out = 1 - normal segment
limit failure if carry out = 0 - stack segment
The
Segment Limit Register tells the MMU the segment limit and
controls which address space is accessed. If the address is within
the siz~ limit of the segment being accessed and the access is to
main memory, the cycle is completed. If the address is beyond the
segment limit a bus error is generated. If the access required is
actually to I/O space, the main memory cycle is terminated and the
I/O cycle is performed. The four control bits in the limit check
register tell the MMU which address space is being accessed.
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