Program Status Words - Xerox 560 Reference Manual

11-bit page address in the accessed element of the memory
map array replaces the 8 high-order bits of the virtual ad-
dress to produce the actual address of the main memory lo-
cation to be used by the instruction (20-bit word address
that is automatically adjusted as required for doubleword,
halfword, or byte operation). See Figure
7.
Note: If the 11-bit page address in the accessed element
of the memory map is all zeros, and an actual ad-
dress is produced that corresponds to a word address
in the range 0 through 15, when the ll-bit page
address is combined with the
9
low-order bits of the
virtual address, the corresponding general register
in the current register block is not accessed. In
this one particular instance a word address in the
range 0 through 15 corresponds to an actual main
memory location rather than a general register.
REAL MEMORY WRITE LOCKS
Additional memory protection, independent of the access
protection, is provided by a write lock and key technique.
A 4-bit write protect lock (WL) is provided for each 512-
word page of actual memory. Thus, for the maximum 1M-
word real memory there would be 2048 4-bit write locks.
Write locks are assigned to pages of actual addresses as
follows:
Actual addresses
X ' 6oo ' -X'7FF'
Actual addresses
X ' 400 ' -X ' 5FF'
Actual addresses
X'200'-X ' 3FF'
Actual addresses
O-X'l FF'
(memory page 0)
Actual
addresses
X'1FEoo'-
X'l FFFF'
(memory
page 255)
Actual
addresses
X'1FCOO'-
X ' 1FDFF'
The write protect locks can be changed only by executing
the privileged instruction MOVE TO MEMORY CONTROL
(see Chapter 3, "Control Instructions").
The write key (a 4-bit field in the PSWs for any operating
program, or in the command doubleword for I/o operations)
works in conjunction with the write lock to determine
whether any program (slave, master-protected, or master
mode) can write into a specific page of main memory ioca-
tions. The write key and lock control access for writing
according to these rules:
1.
A lock value of 0000 means that the corresponding
memory page is unlocked; write access to that page is
permitted independent of the key value.
28 Main Memory
2. A key value of 0000 is a "skeleton
II
key that will open
any lock; thus write access to any memory page is per-
mitted independent of its lock value.
3. A lock value other than 0000 for a memory page per-
mits write access to that page only if the key value
(other than 0000) is identical to.the lock value.
Thus a program can write into a given memory page if the
lock value is 0000, if the key value is 0000, or if the key
value matches the lock value.
Note: The memory access protection feature operates dur-
ingvirtualaddressing modes and on virtual addresses,
whereas the memory write protection feature always
operates on actua
I
memory addresses. Thus, if the
memory access protection feature is invoked (that
is, if the basic processor is operating in the slave
mode or the master-protected inode and is usi ng the
memory map), the access protection codes are ex-
amined when the virtual address is converted into
an actual address. Then the lock and key are ex-
amined to determine whether the program (master,
master-protected, or slave mode) is allowed to alter
the contents of the main memory location correspond-
ing to the final actual address. If an instruction at-
tempts to write into a write-protected memory page,
the basic processor aborts the instruction, and traps
to location X 140 1
,
the "nonallowed operation" trap
(see "Trap System ", later in this chapter). If an
I/O procedure attempts to write into a write-
protected memory page, the write lock violation bit
in the lOP sta tus byte is set, and can be tested by
the AIO, TIO, and TDV instructions.
PROGRAM STATUS WORDS
The critical control conditions of the basic processor are de-
fined within 64 bits of information collectively referred to
as the program status words (PSWs). The current PSWs may
be considered as one 64-bit internal basic processor register,
although they actually exist as a collection of separate reg-
isters and flip-flops (see Figure 2 appearing earlier in this
chapter). When stored in memory, the PSWs have the fol-
lowing format:
They may be optionally followed by an additional two words
with the following format: