Cray CRAY-1 - Exchange Package Management

EXCHANGE PACKAGE MANAGEMENT
Each 16-word exchange package resides in an area defined during system
dead start that must lie within the lower 4096 words of memory. The
package at address 0 is that of the monitor program. Other packages
provide for object
programs and monitor tasks. These packages lie
outside of the field lengths for the programs they represent as
determined by the base and limit addresses for the programs. Only the
monitor
program has a field defined so that it can access all of memory
including the exchange package areas. This allows the monitor program
to define or alter all exchange packages other than its own when it is
the currently active exchange package.
Proper
management of exchRnge packages dictates that a non-monitor
program always exchange back to the monitor program that exchanged to
it. This assures that the program information is always swapped back
into its proper exchange package.
Consider the case
v/here exch~nge packages exist for programs A, 8, and C.
Program A is the monitor program, program B is a user program, and program
C is an interrupt processing program.
The monitor program, A, begins an execution interval following dead start.
No interrupts can terminate its execution interval since it is in monitor
I mode
t
. The monitor program voluntarily exits by issuing a 004 exit
instruction. Before doing so, however, it sets the contents of the XA
register to point to Bls exchange package so that B will be the next
program to execute and it sets the exit address in 8's exchange package
to point back to the monitor.
I
The exchange sequence to B causes the exit address from Bls exchange
package to be entered in the XA register. At the same time, the exchange
address in the XA register goes to Bls exchange package area along with all
other program parameters for the monitor program. When the exchange is
complete, program B begins its execution interval.
t Assumes Monitor Mode Interrupt Option is not present. Refer to descrip-
tion of M
register.
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