Time-Sharing Features - Xerox 560 Reference Manual

Many operations are performed in floating-point format
and on strings of characters. Other typical characteristics
include decimal arithmetic operations, binary to decimal
number conversion (for printing or display), and high sys-
tem i nput/ output transfer rates.
General-purpose features are described in the following
paragraphs.
Floating-Point Hardware. Both short (32-bit) and long
(64-bit) formats are available in the floating-point in-
structions. Under program control, the user may select
optional zero checking, normalization, floating-point
rounding and significance checking. Significance check-
ing permits use of short floating-point format for high pro-
cessing speed and storage economy and of long floating-
point format when loss of significance is detected.
Decimal Arithmetic Hardware. Decimal arithmetic instruc-
tions operate on up to 31 digits plus sign. This instruction
set includes pack/unpack instructions for converting to/from
the packed format of two digits per byte, and a generalized
edit instruction for zero suppression, check protection, and
formatting, with punctuation to display or print it.
Indirect Addressing. Indirect addressing faci litates table
linkages and permits keeping data sections of a program
separate from procedure sections for ease of maintenance.
Displacement Indexing. Indexing by means of a IIfloating li
displacement permits accessing a desired unit of data with-
out considering its size. The index registers automatically
align themselves appropriately; thus, the same index reg-
ister may be used on arrays with different data sizes. For
example, in a matrix multiplication of any array of full
word, single-precision, fixed-point numbers, the results
may be stored in a second array as double-precision num-
bers, using the same index quantity for both arrays. If an
index register contains the value of k, then the user always
accesses the kth element, whether it is a byte, halfword,
word, or doubleword. Incrementing by various quantities
according ro daro size is nor required; instead, increment-
ing is always by units in a continuous array table regardless
of the size of data element used.
Instruction Set. More than 100 major instructions permit
short, highly optimized programs to be written. These are
rapidly assembled and minimize both program space and
execution time.
Translate Instruction. The Translate instruction permits
rapid translation between any two 8-bit codes; thus, data
from a variety of input sources can be handled and re-
converted easi iy for output.
Conversion Instructions. Two generalized conversion in-
structions provide for bidirectional conversions between
internal binary and any other weighted number system,
including BCD.
4 Time-Sharing Features
Call Instructions. These four instructions permit handling
up to 64 user-defined subroutines, as if they were built-in
machine instructions. Call instructions also gain access to
specified operating system services without requiring its
i nterventi on.
Interpret Instruction. The Interpret instruction simplifies
and speeds interpretive operations such as compilation, thus
reducing space and time requirements for compilers and
other interpretive systems.
Four-Bit Condition Code. Checking results is simplified by
automatically providing information on almost every instruc-
tion execution, including indicators for overflow, under-
flow, zero, minus, and plus, as appropriate, without
requiring an extra instruction execution.
Direct Input/Output (DIO). Direct input/output faci li-
tates in-line program control of asynchronous or special-
purpose devices. This feature permits information to be
transmitted directly to or from general-purpose registers.
Multi lexor Input/Out ut Processor (MIOP). Once initia-
lized, I 0 processors operate independently of the basic
processor, freeing it to provide faster response to system
needs. An MIOP requires minimal interaction with the
basic processor. I/O command doublewords permit both
command chaining and data chaining without intervening
basic processor control. I/o equipment speeds range from
slow rates involving human interaction (teletypewriter, for
example) to transfer rates of rotating memory devices of
over 750,000 bytes per second. Peripheral controllers at-
tached to an MIOP may be operated simultaneously.
Rotating Memory Processor (RMP). An RMP supports up to
15 disk drives, one at a time, permitting large capacity,
high transfer rate files. Dual access (between 2 RMPs) op-
tion is available.
TIME -SHARING FEATURES
Ti me-shari ng is the abi Ii ty of a system to share its tota I
resources among many users at the same time. Each user
may be performing a different task, requiring a different
share of the available resources. Some users may be on-
line in an interactive, IIconversational li mode with the
basic processor while other users may be entering work to
be processed that requires only final output.
Time-sharing features are described in the following
paragraphs.
Rapid Context Saving. When changing from one user to
another, the operating environment can be switched quickly
and easily. Stack-manipulating instructions permit storing
in a push-down stack of 1 to 16 general-purpose registers by
a single instruction. Stack status is updated automatically
and information in the stack can be retrieved when needed