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by units in a continuous array table no matter which size
of data element is used.
Powerful Instruction Set. The availability of more than
100 major instructi ons results in programs that are short,
rapidly assembled, and quickly executed.
Translate Instruction.
This instruction permits rapid trans-
lation between any two 8-bit codes (such as EBCDIC to
ANSCII); thus data from a variety of input sources can be
easi Iy handled and reconverted for output.
Conversion Instructions. Two generalized conversion in-
structions provide for bidirectional conversions between
internal binary and any other weighted number system,
including BCD.
Call Instructions. Four instructions permit handling up to
64 user-defined subroutines (as if they were built-in
machine instructions) and gaining access to specified oper-
ating system services without requiring its intervention.
Interpret Instruction. This instruction simplifies and speeds
interpretive operations such as compi I ing, thus reducing the
space and time requirements for compilers.
Four-Bit Condition Code. This feature simplifies the
checking of results by automatically providing information
on almost every instruction execution (including indicators
for overflow, underflow, zero, minus, and plus, as appro-
priate) without requiring an extra instruction execution.
TIME -SHARING FEATURES
Time-sharing is the abi lity of a computer system to share
its resources among many users at the same time. Each
user may perform a different task that requires a different
share of the avai lable resources and, in many instances,
each may be on-line in an interactive ("conversational")
mode with the computer. Other users may enter work to be
batch processed. The SIGMA 6 system provides for the fol-
lowing time-sharing computer features.
Rapid Context Saving. When changing from one user to
another, the operating environment can be switched quickly
and easi Iy. Stack-manipulating instructi ons permit from
one to 16 general-purpose registers to be stored in a push-
down stack by a single instruction - with automatic updating
of stack status information - and to be retrieved (again, by
a single instruction) when needed. The current program
status doubleword (which contains the entire description of
the current user's environment and mode of operation) can
be stored anywhere in memory and a new program status
doubleword loaded, all with a single instruction.
Multiple Register Blocks. The optional avai lability of up
to 32 blocks of 16 general-purpose registers further improves
response time by reducing the need to store and load regis-
ter blocks. As needed, ea~h user can be assigned a distinct
block; the program status doubleword automatically points
to the currently appl icable register block.
6
Time-Sharing/Multiuse Features
User Protection. The slave mode of operati on restricts each
user to his own set of instructions while reserving to the
operating system those instructions that could, if used in-
~
correctly, destroy another user's prog:am. A memory
acce~
protection system prevents any user from accessing storage
areas other than those assigned to him. This access protec-
tion permits the user to access certain areas for reading only,
such as those containing public subroutines, whi Ie preventing
him from reading, writing, or accessing instructions in areas
set aside for other users.
Storage Management. SIGMA 6 memories are available in
I'
seven sizes (from 32,768 to 131,072 words) to provide the ca-
pacity needed, while assuring potential for expansion. To
assure efficient use of available memory, the memory map
hardware permits storing a user's program in fragments (as
small as 512 words) wherever space is available; yet, all
fragments appear as a single, contiguous block of storage at
execution time. The memory map also automatically and
dynamically handles program relocation, so that the pro-
gram appears to be stored in a standard way at execution
time (even though it may actually be stored in a different
set of locations each time it is brought into memory).
The
memory map for the full-sized SIGMA 6 memory is provided
no matter how sma II the actua I memory may be. Th us, the
system can always address a virtual memory of 131,072 words
regardless of physical memory size.
Input/Output Capability. Sigma 6 can control up to eight I
input/output processors (of two types) in various combi-
nations. Each multiplexor I/O processor or MIOP expansiort-"
option can have up to 24 standard-speed I/O channels op-
erating simu Itaneously; selector I/O processors can have any
one of up to 32 high-speed I/O devices operating on each
processor. The I/O processors operate semi-independently
of the central processor, leaving it free to provide faster
response to overall system needs.
Nonstop Operation. A watchdog timer assures that the
system conti nues to operate even if certain special I/O
capabilities are used with special devices that can cause
delays or halts if they fail. Multiple real-time clocks with
varying resolutions permit establishing several independent
time bases, thus allowing flexible allocation of time slices
to each user.
MULTIUSE FEATURES
As implemented in the SIGMA 6 system,
II
multi use
II
com-
bines two or more computer applicati on areas. The most
difficult computing problems are associated with real-time
applications. Simi larly, the most difficult multiuse prob-
lems are associated with time-sharing applications that
include one or more real-time processes. SIGMA 6 sys-
tem design is especially suited for a mixture of applica-
tions in a multiuse environment. Many of the hardware
features that are required for specific application areas
are equally useful in others, although in different ways.
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