Ausam/sys/conf/source/sysld.1

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.th SYSLD VIII 07/06/77
.sh NAME
	sysld \*- system loader for pdp-11/40
.sh SYNOPSIS
	sysld proto
.sh DESCRIPTION
.it sysld
is a special loader for unix systems on pdp-11/40's that allows
more efficient use of the limited addressing space available for the
system resident code.  Certain code and data may be specified to be
permanently addressible; other data and code is addressible only
when needed. One of the kernal segmentation registers - KISA5 -
is reserved for addressing these items.
.s3
sysld requires the division of the unix system into
.it segments
-- logically related units of text and data which must be simultaneously
addressible. One segment is permanently addressible -- the root
segment -- and is limited to a 20Kw total size.  The other segments
are each limited to 4Kw.
.s3
The
.it proto
describes the division of the system.
It has the format of a shell command file, except that
newlines are not escaped.
.s3
When a transfer is made to an \fIautoload\fR point in one of the
non-root segments, control is actually passed to a
transfer vector in the root segment.  This transfer vector
saves the current value of the page 5 address register on the stack, and loads the
address register with the value appropriate to the called function.  It adjusts
the stack so that the arguments to the function appear correct,
and then calls the function. When the function returns, the
transfer vector restores the page 5 address register to its saved value, and
then returns to the caller.
.s3
The page 5 register is also saved in a special word in the u-table
on calls to savu,  and restored on calls to retu.  This allows
device drivers and other routines that must call \fIsleep\fR
to reside in non-root segments.
.s3
The overhead of calling a non-resident function is directly related
to the number of arguments which the function receives.  It is
small in all cases though -- probably equivalent in time to
a call to \fIcsv\fR.
.s3
It may be seen that certain non-stack disciplines in function
invocation, such as are provided in UNIX via \fIaretu\fR could
be incompatible with the above scheme.  However, these calls
may be analyzed on a case-by-case basis as follows:
.br
1) Each aretu is proceeded by a retu, so that the page 5
register is always restored from the time of the last
call to savu.
.br
2) \fItrap\fR and \fIclock\fR, since their arguments are
not really called by value, cannot be placed in non-root
segments. Hence the jump in swtch to \fIqsav\fR (trap) is
proper.
(No other functions in the
standard UNIX distribution or in the systems I have seen modified
at N.S.W. use this 'call by value/result' trick.
However, some systems have changed certain device drivers, like the
terminal handlers, so that signals, when received, are immediately passed
to the running process if appropriate.  This is senseless. However,
it means that the interrupt routines that take these arguments on the
stack cannot be in non-root segments.)
.br
3) The return from \fInewproc\fR is made in the child through
the \fIssav\fR save area.
However, newproc calls swtch directly after setting up the ssav return,
and swtch saves the page 5 address register and restores it before the
return. Hence there is no problem.
.br
4) Certain swap routines make inessential use of the SSWAP flag.
They can either be made resident, or recoded to avoid this use.
.s3
For those unwilling to go through the above logic, it can be noted that
all of the routines in sys1.c through sys4.c, and all of the
device drivers, meet all of the criteria for non-root
segments.
.s3
The calling conventions may be seen to allow functions in one non-root segment
to freely call functions in other non-root segments.
Function calls withing a segment generally do not pass through the
transfer vector to save time.  However, addresses of functions used
within a segment (such as arguments to timeout) do refer to the transfer vector.
.s3
Sometimes device drivers or other
routines which can be made non-root segments contain common segments
which must always be addressible.
(For example, \fItty\fR structures in a particular tty driver which must
be available to the restart routine in tty.c on a timeout.)
Sysld will place all common segments referenced in more than one segment
in the root; provisions exist for forcing a common segment referenced
in only one segment into the root as well. (-C below)
.s3
Sysld also has provisions for providing 'once-only' code facilities
for the system start-up.  All code designated as once-only is
made into a segment which is placed 2*USIZE+2 memory blocks after
the end of the system at run-time. This provides space for the
u table for the scheduler, and the u table and initial core allocation
for the initialization process. Once-only code is therefore defined as
code which may be destroyed once the initialization process starts executing.
.s3
Sysld takes the standard linker flags for restricting symbol
table information, signalling libraries, etc. (-o, -u -e, -l, -x,
-X, -s, -S). Its special flags are as follows:
.br
.lp +10 5
-L	Provide a load map for the segmented system.
.lp +10 5
-c	List the segment in which all common segments appear, and their size.
.lp +10 5
-N	All segments following this are once-only.
.lp +10 5
-O	A non-root segment is given by the following files up to the next
segment description.
.lp +10 5
-R	The root segment follows.
.lp +10 5
-A	A list of autoload points follows. Each symbol
must have the number of arguments to it specified in the form symbol:n. If the ':n' is
omitted, 0 is assumed.
.lp +10 5
-C	A list a common segments that are to forced into the root follows.
.lp +10 5
-V	Symbol are placed in the symbol table for all of the autoload
entry points.
These symbols consist of the appropriate C symbol with the underscore
replaced with a dot.
.in 0
.s3
Sysld defines the following symbols analogous to '_end':
.lp +12 6
_eover	address at which the non-root segment text ends.
.lp +12 6
_etoa	virtual address to which the non-root segment text is to be moved.
.lp +12 6
_eto	block number to which non-root segment text is to be moved.
.lp +12 6
_eres	address at which non-once-only segment text ends.
.lp +12 6
_eu	block number of scheduler u-table
.in 0
.s1
m40.s has been changed to accomplish the required initial-core setup.
The calls to clearseg in main have also been eliminated. (They were
unnecessary, and would have eliminated the once-only code).
.s3
.br
Relatively few changes would be necessary to allow sysld to run on
pdp-11/45-style systems.  The vectors would have to be placed in the
data rather than text segment, in order to be addressible as both
data and instruction.  A pair of I/D segmentation registers would have
to be used in place of KISA5.  Sysld would also have to perform the
functions of sysfix.