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Principles of Operation
Prime Proprietary
(addresses
'2000
-
'2777) or sector 0 (addresses '0000 - '0777).
A
machine that uses 16S mode can access the first 16k words of memory in
this way;
32S means the machine can access the
first
32K
words
in
this manner.
Relative
addressing (R mode) is an extension of S mode.
Instructions
can access memory addresses in sector 0 directly, as in
S
mode.
In
addition, instructions can access addresses in other sectors, based on
16-bit
address
codes stored in the program counter (P register).
An
instruction can access any memory address in any sector.
In 64R
mode
a
machine
can
access
any
address
in
the first 64K words of real
memory .
.
Virtual (V mode) and Identity (I mode) addressing
are
based
on
the
concept
of
segmented
virtual memory, and 32-bit internal registers.
In V mode, instructions reference
28-bit
virtual
memory
addresses,
instead
of 22-bit real memory addresses.
Instructions can access 64k
words of virtual memory (8M' real memory) in 64V mode.
In I mode (32-bit Identity mode), each memory word is 32 bits, instead
of 16 bits used in S, R, and V modes.
Instructions
can
access'
the
first
64K
words of memory directly, plus 64K words of virtual memory
(8Mb real memory).
4.2.5.1 Absolute Addressing
The CPU has many operating
modes,
but
only
two
memory
addressing
modes.
In absolute memory addressing mode, real memory addresses are
generated by
and
for
all
program
instructions.
If
the .cPU
is
operating
in
S or R (sectored or relative) modes, only the first 64K
words of real memory are addressable in the absolute memory
accessing
mode.
However, 64V or 32I modes of operation, taking advantage of the
CPU's
32-bit
internal
registers, may use all 22 real memory address
bits (BMA95~ BMA16) to access up to 4 million
words
of
real
memory
(8
Mb).
4.2.5.2 Virtual (Segmented/Paging) Addressing
In
this
mode
of memory accessing, memory addresses generated by and
for the computer program are virtual.
That is, the addresses
do
not
point directly to physical memory.
Virtual
memory
is
also called mapped memory.
Prime divides virtual
memory into segments.
Think of the segments as chapters
in
a=
book,
Some
segments
are
shared
by all users ('0000 to '3777) and contain
PRIMOS, compilers, shared libraries, etc.
The rest
of
the
segments
('4000 to '7777) are divided between individual users.
Each segment (or chapter) contains up to 64 pages.
Each page contains
1024 words (2Kbytes) and is equal to one physical disk record.
The
28-bit virtual address contains the segment number (12-bit), page
number within the segment (6-bit), and the word number within the page
(10-bit).
Through a series of hardware and software
mapping
tables,
the
segment
number
and
page
number are translated into the 12-bit
physical page number (PPN).
Appending the PPN to the
10-bit
virtual
word
number
gives the 22-bit physical memory address, as illustrated
in Figure 4-6.
Once the physical address is
determined,
it
can
be
070-C
4-
8
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