AT&T 3B2/300 Technical Reference Manual page 349

FUNCTIONAL D E S C R I P T I O N - - - - - - - - - - - - - - - - - - - - - - - - - -
Stack Pointer Register (r12)
The Stack Pointer (SP) register contains the current 32-bit address of the top of the execution stack.
This is the memory address of the next place where an item can be stored (pushed) on the stack or the
last place where an item was retrieved (popped) from the stack. The SP implements a Last-In-First-Out
(LIFO) queue for efficient subroutine linkage and local variable storage. The contents of Stack Pointer
register (r12) are part of the error report output by the /etc/errdump command. This register is
identified as "osp" in the error report.
Process Control Block Pointer Register (r13)
The Process Control Block Pointer (PCBP) register contains the 32-bit address of the Process
Control Block (PCB) for the current process. The PCBP register is kernel level privileged (can only be
written when the CPU is in the kernel mode). The PCB contains all switchable process context
collected into a compact form for ease of movement between system memory and privileged internal
registers. This context consists of the initial and current contents of the processor status word, program
counter, and stack pointer; the last contents of registers rO through rlO; boundaries for an execution
stack; and block move specifications for the process. The contents of register r13 are part of the error
report output by the /etc/errdump command. This register is identified as "pcbp" in the error report.
Interrupt Stack Pointer Register (r14)
The Interrupt Stack Pointer (ISP) register (r14) contains the 32-bit memory address of the top of the
interrupt stack. This stack is used when an interrupt request is received. The interrupt stack is also
used by the Call Process (CALLPS) and Return To Process (RETPS) instructions. The ISP register is
kernel level privileged. The contents of register r14 are part of the error report output by the
/etc/errdump command. This register is identified as "isp" in the error report.
Program Counter Register (r15)
The Program Counter (PC) register (rlS) contains the 32-bit memory address of the instruction
being executed or, on instruction completion, contains the starting address of the next instruction to be
executed. The contents of register rlS are part of the error report output by the /etc/errdump
command. This register is identified as "ope" in the error report.
General-Purpose Kernel Registers (r31-r24)
These eight registers can be used for accumulation, addressing, or temporary storage. They are
kernel-level privileged and can be used in any addressing mode by any privileged program.
3-96 TECHNICAL REFERENCE MANUAL