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Instruction in memory:
Instruction in instruction register:
The 8 high -order bi ts of the reference address are
replaced with page address Z from memory map:
Actual address of memory location
that contains the direct address:
Di rect address in memory:
Indirect addressing replaces reference
address wi th di rect address:
Halfword operation indexing alignment:
Effective virtual address:
The 8 high-order bits of the effective address are
replaced with page address N from memory map:
Final memory address, which is the actual address of
halfword location containing the effective halfword:
II I I
II II
Figure 5.
Generation of Actual Memory Addresses
The write-key (a 2-bit field in the arithmetic and control
unit) works in conjunction with the lock storage to deter-
mine whether or not the program (whether slave or master)
can write into a specific page of core memory locations.
The keys and locks control access for wri ti ng, accordi ng to
the following rules:
A lock value of 00 means that the corresponding mem-
ory page is "unlocked"; write access to that page is
permitted independent of the key value.
A key value of 00 is a "skeleton
ll
key that wi II open
any locki thus, write access to any memory page is
permitted independent of its lock value.
A lock value other than 00 for a memory page permits
write access to that page only if the key value is
identical to the lock value.
16
Memory Address Control
Thus, a program can write into a given memory page if
the lock value is 00, if the key value is 00, or if the key
value matches the lock value.
Note that the memory access protection feature is pro-
vided with the memory map and operates on virtual ad-
dresses, whereas the memory write proctection feature
operates on actual memory addresses.
Thus, if the ac-
cess protection feature is invoked (that is, the CPU is
in the slave mode and is using the memory map), the access
protection codes are examined at the time the virtual ad-
dress is converted into an actual address. Then, the locks
and keys are examined to determine whether or not the
program (master or slave) is a lIowed to alter the content<
of the core memory location corresponding to the final
actual address. If an instruction attempts to write into
a write-protected memory page, the computer aborts
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