Main Memory; Memory Unit - Xerox 550 Reference Manual

instruction word. Hence, memory reference, indirect
addressing, and indexing are not required.
Bits
o
1-7
8-11
12-31
Description
Bit position 0 must be coded with a zero.
If
it
contains a one, the instruction is interpreted as
being nonexistent. (See "Trap System", later in
this chapter.)
Operation code. This 7-bit field contains the code
that designates the operation to be performed.
When the basi c processor encounters any i mmedi-
ate operand operation, it interprets bits 12-31 of
the instruction word as an operand. These are the
immediate operand operation codes:
Operation
Code
X'02'
X'20'
X'21'
X'22'
X'23'
Instruction
Name
Load Conditions
and Floating Con-
trol Immediate
Add Immediate
Compare Immediate
Load Immediate
Multiply Immediate
Mnemonic
LCFI
AI
CI
LI
MI
R field. This 4-bit field designates one of the
first 16 general registers in the current general
register block. The register may contain another
operand and/or be designated as the register in
which the results of the operation are to be
stored or accumulated.
Operand. This 20-bit field contains the immedi-
ate operand. Negative numbers are represented
in two's complement form. For arithmetic opera-
tions bit 12 (the sign bit) is extended by duplica-
tion to the left through bit position 0 to form a
32-bit operand.
MAIN MEMORY
The memory system comprises memory units, memory inter-
faces (MIs), and memory buses. Figure 4 illustrates the re-
lationships among these components.
The primary technology for main memory is magnetic core.
The maximum physical storage is 256Kwords. Memory units
can be interleaved on a two-way interleave basis. Each
memory unit is provided with a set of starting address
switches on the Configuration Control Panel (see Chapter 6)
together witha two-position switch that selects one of two
14 Main Memory
possible clock and power sources. Memory units may con-
tain two, four, or six ports, which have a fixed priority
order for the resolution of contention problems.
The following sections describe the organization and opera-
tion of the memory system. Also described are the various
modes and types of addressing, including indexing.
MEMORY UNIT
Main memory is divided physically and logically into one
to eight module assemblies called memory units. Because
the memory unit is a logical component that contains all the
functions available in the entire memory, the minimum mem-
ory is one memory unit. The minimum storage capacity per
memory unit is 16K words; the maximum is 32K words. A
memory location stores a word of 36 bits; the first 32 bits are
information and the last 4 are byte parity bits (the latter
being unavailable to the program). Each memory unit com-
prises a specific storage capacity, drive and sense circuits,
a set of operational registers (address, data, and status), a
set of write lock control registers for 32K words of memory,
and a timing and control unit.
CORE MEMORY MODULES
Core memory modules (CMMs) provide a storage faci lity of
standard modules (see Figure 4).
MEM ORY DRIVER
The memory driver in each memory unit performs all memory
operations except storage (provided for by the CMMs) and
the few operations performed by the ports. The major func-
tions of the memory driver are:
1 • Store address word.
2. Store data-in and data-out words during memory
cycles.
3. Store write locks in special memory (other than CMMs).
4. Perform parity generation and checking on address and
memory bus data words, and on core memory module
words.
5. Generate and store status words.
6. Control and time all transfers of address words, data
words, status words, write locks, and write key among
the ports, CMM, and the storage registers.
7. Centre! end time o!! date,
pcr:f';1
and control signals
issued to the memory bus.
8. Accept one of two or more simultaneous memory re-
quests on the basis of port positional priority and other
priority status information such as "high priority" and
"memory reserved ".