.TH CHOVERLAY 2 
.UC
.SH NAME
choverlay \- change overlay
.SH SYNOPSIS
.B choverlay(ovno)
.br
.B int ovno;
.SH DESCRIPTION
.I Choverlay
is an "invisible" system call used by overlaid (type 0430 or 0431)
programs to remap a fixed segment of the process's text address space
to one of up to seven different overlay segments. These segments are
determined by the 
.IR ld (1)
utility at the time of program creation.  Similarly,
.IR ld (1)
inserts the appropriate 
.I choverlay
calls to effect the overlaid execution of the process.
.PP
Notice that the
.I choverlay
system call is intended only to be used by the overlay mechanism. In
fact, it is impossible to use this system call in nonoverlaid programs
because of its implementation. Instead of using the `sys' or `trap' instruction
as other system calls, this call redefines the `emt' instruction as the
overlay call. In ordinary (nonoverlaid) programs this ``emulator trap
instruction'' generates a SIGEMT exception.  In overlaid programs the
instruction switches to the overlay indicated in general register zero (r0).
Overlays are numbered 1-7, with 0 indicating that no overlay is to be mapped.
If the number specified corresponds to
an overlay that is unused (has zero length),
no overlay will be mapped.
If the overlay number is outside of the range of 0-7
a SIGEMT exception will be generated.
.PP
Overlay address regions are mapped by one or more segmentation registers
in the user text address space, begining with the segmentation
register following regular text space.
.SH WARNING
The
.I choverlay
system call is meant to be executed from within the normal text space.
Unpredictable results will occur if this call is attempted from within
an overlay.
.SH "SEE ALSO"
ld(1), signal(2), a.out(5)
.SH ASSEMBLER
.br
(overlay number r0)
.br
\fBemt\fP		/ text address space contents will change