Xerox 550 User Manual

MASTER MODE
The master/slave control bit (bit 8 of the PSWs) must con-
tain a zero for the basic processor to operate in master
mode. In this mode the basic processor can perform all of
its control functions and can modify any part of the system.
The restrictions upon
the basic processor1s operations in this
mode
are those imposed by the write locks on certain pro-
tected parts of memory. It is assumed that there is a res-
ident operating system (operating in the master mode) that
con tro Is and supports the opera ti on of other programs (wh i ch
may be in the master, master-protected, or slave mode).
MASTER-PROTECTED MODE
The master-protected mode of operation provides additional
protection for programs that operate in the master mode. The
master-protected mode occurs when the basic processor is
operating in the master mode with the memory map in effect
and the mode altered control bit (bit 61 of the PSWs)is on.
In this mode the memory protection violation trap occurs
(location X 140
1
,
with CC4 = 1), as it does in all mapped
slave programs, if a program makes a reference to a virtual
page to which access is prohibited by the current setti ng of
the access protection codes.
SLAVE MODE
The slave mode of operation is the problem-solving mode
of the basic processor. In this mode, access protection
codes apply to the slave mode program if mapping is in ef-
fect, and all "privi leged" operations are prohibited. Priv-
ileged operations are those relating to input/output and to
changes in the fundamental control state of the basic pro-
cessor. All privi leged operations are performed in the
master or master-protected mode by a group of privileged
instructions. Any attempt by a program to execute a priv-
i leged instruction whi Ie the basic processor is in the slave
mode results in a trap. The master/sla·:e mode control bit
(bit 8 of the PSWs) can be changed when the basic processor
is in the master or master-protected mode. Nevertheless,
a slave mode program can gain direct access to certain ex-
ecutive program operations by means of CAll instructions.
The operations available through CAll instructions are es-
tablished by the resident operating system.
MAPPED
MODE
Although the memory map is located in the Memory Inter-
face (MI), it functions as part of the basic processor. The
basic processor communicates with memory through the MI.
Mapping is effective for all the words of real memory, and
is invoked when bit 9 (MM) of the PSWs contains a one.
Memory mopping generates rea! page addresses from v:rtuc!
addresses. The memory map can be loaded with either
11-bit real page addresses or 8-bit real page addresses by
means
of the MOVE MEMORY CONTROL (MMC) privi leged
instruction (see Chapter 3, "Control Instructions "). Eleven-
bit rea I page addresses are always provided for in the map,
thus
if 8-bit real page addresses are generated, the three
12 Basic Processor
high-order bits contain zeros. The memory map always maps
17-bit virtual addresses into 20-bit real addresses (see
"Memory Address
Control ", later in this chapter for a dis-
cussion of how the map is used).
UNMAPPED MODE
When the basic processor is operating in the unmapped mode,
there is a direct one-to-one relationship between the effec-
tive virtual address of each instruction and the actual ad-
dress used to access main memory. (See "Real Addressing",
later in this chapter.)
INFORMATION FORMAT
Nomenclature associated with digital information within the
computer system is based on functional and/or physical at-
tributes. A "word" may be either a 32-bit instruction word
or a
32-bit data word.
The bit positions of a word are numbered from 0 through 31
as follows:
A word
can be divided into two 16-bit parts (halfwords) in
which the bit positions are numbered from 0 through 15 as
follows:
A word
can also be divided into four 8-bit parts (bytes) in
which the bit positions are numbered 0 through 7 as follows:
Two words can be combined to form a 64-bit element (a
doubleword) in
which the bit positions are numbered 0
through 63 as follows:
In fixed-point binary arithmetic each element of information
represents numerical data as a signed integer (bil 0 repre-
sents the sign, remaining bits represent the magnitude, and
the binary point is assumed to be just to the right of the
least significant or rightmost bit). Negative values are
represented in two1s complement form. Other formats re-
quired for floating-point and decimal instructions are de-
scribed in Chapter 3.

Xerox 550 Specifications

General

Brand	Xerox
Model	550
Category	Printer
Language	English