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7.5.1.1
Procedure Entries
A procedure entry in the system procedure table specifies a procedure's location and type. Each
entry is one word in length and consists of an address (IP) field and a type field. The address field
gives the address of the first instruction of the target procedure. Since all instructions are word
aligned, only the entry's 30 most significant bits are used for the address. The entry's two
least-significant bits specify entry type. The procedure entry type field indicates call type:
system-local call or system-supervisor call
different actions depending on the type of call selected.
Table 7-1. Encodings of Entry Type Field in System Procedure Table
Encoding
00
01
10
11
1. Calls with reserved entry types have unpredictable behavior.
7.5.1.2
Supervisor Stack Pointer
When a system-supervisor call is made, the processor switches to a new stack called the supervisor
stack, when not already in supervisor mode. The processor gets a pointer to this stack from the
supervisor stack pointer field in the system procedure table
ization sequence and caches the pointer internally. Only the 30 most significant bits of the
supervisor stack pointer are given. The processor aligns this value to the next 16-byte boundary to
determine the first byte of the new stack frame.
7.5.1.3
Trace Control Bit
The trace control bit (byte 12, bit 0) specifies the new value of the trace enable bit in the PC register
(PC.te) when a system-supervisor call causes a switch from user mode to supervisor mode. Setting
this bit to 1 enables tracing in the supervisor mode; setting it to 0 disables tracing. The use of this bit
is described in
section 9.1.2, "PC Trace Enable Bit and Trace-Fault-Pending Flag" (pg.
(Table
7-1). On a system call, the processor performs
Call Type
System-Local Call
1
Reserved
System-Supervisor Call
1
Reserved
(Figure
7-4) during the reset initial-
PROCEDURE CALLS
9-3).
7-17
7
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