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researchv10 Dan Cross

.TH MEM 4
.CT 2 sa
.SH NAME
mem, kmem, kUmem, mtpr, fineclock \- memory and VAX processor registers
.SH DESCRIPTION
.lg
.I Mem
is a file that is an image of the main memory
of the computer.
It may be used to examine
(and even to patch) the system.
Byte addresses in
.I mem
are interpreted as physical memory addresses.
References to non-existent locations return errors.
.PP
.IR Kmem
and
.I kUmem
are like
.IR mem ,
but access kernel-mode virtual memory.
.I KUmem
promises that reads and writes will be done
in two-byte quantities;
this is convenient for UNIBUS accesses.
.PP
.I Mtpr
accesses
VAX internal processor registers.
Each register is 4 bytes long;
register
.I n
may be read or written at address
.IR n \(**4.
.PP
.I Fineclock
reads a high-resolution clock.
Reading four bytes returns a 32-bit unsigned integer
representing the number of microseconds since
the epoch 00:00:00 GMT, Jan. 1, 1970,
with high-order bits discarded.
.PP
.IR Mem ,
.IR kmem ,
.IR kUmem ,
and
.IR mtpr
have minor device numbers
0, 1, 3, and 5.
.I Fineclock
is a separate driver;
the minor device number is ignored.
.SH FILES
.EX
.F /dev/mem
.F /dev/kmem
.F /dev/kUmem
.F /dev/mtpr
.F /dev/fineclock
.EE
.SH SEE ALSO
.IR time (2)
.br
VAX Hardware Handbook
.SH BUGS
Examining and patching device and processor registers may
give unexpected results when read-only or write-only
bits are present.
.br
An attempt to read a nonexistent processor register
returns 0
instead of an error.
.br
The
.I fineclock
counter overflows every hour or so.
It is only as precise as the hardware;
hence it is inaccurate on a MicroVAX.

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