GMS30C2116 Datasheet, PDF (40/322 Pages) Hynix Semiconductor – USERS MANUAL

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CHAPTER 1
1.7 Stack
A runtime stack, called stack here, holds generations of local variables in last-in-first-out
(LIFO) order. A generation of local variables, called stack frame or activation record, is
created upon subprogram entry and released upon subprogram return.
The runtime stack provided by the architecture is divided into a memory part and a register
part. The register part of the stack, implemented by a set of 64 local registers organized as
a circular buffer, holds the most recent stack frame(s). The current stack frame is always
kept in the register part of the stack. The frame pointer FP points to the beginning of the
current stack frame (addressed as register L0). The frame length FL indicates the number
of registers (maximum 16) assigned to the current stack frame. The stack grows from low
to high address. It is guarded by the upper stack bound UB.
The stack is maintained as follows:
¡ Ü A Call, Trap or Software instruction increments the FP and sets FL to six, thus creating
a new stack frame with a length of six registers (including the return PC and the return
SR).
¡ Ü An exception increments the FP by the value of FL and then sets FL to two.
¡ Ü A Frame instruction restructures a stack frame to include (optionally) passed parameters
by decrement the FP and by resetting the FL to the desired length, and restores a reserve
of 10 local registers for the next subprogram call. If the required number of
registers + 10 do not fit in the register part of the stack, the contents of the differential
(required + 10 - available) number of local registers are pushed onto the memory part of
the stack. A trap to Frame Error occurs after the push operation when the old value of
the stack pointer SP exceeded the upper stack bound UB. The passed parameters are
located from L0 to the required number of register to be saved passed parameters.
Note: A Frame instruction must be executed before executing any other Call, Trap or
Software instruction or before the interrupt-lock flag L is being cleared, otherwise the
beginning of the register part of the stack at the FP could be overwritten without any
warning.
¡ Ü A Return instruction releases the current stack frame and restores the preceding stack
frame. If the restored stack frame is not fully contained in the register part of the stack,
the content of the missing part of the stack frame is pulled from the memory part of the
stack.
For more details see the descriptions of the specific instructions.
When the number of local registers required for a stack frame exceeds its maximum length
of 16 (in rare cases), a second runtime stack in memory may be used. This second stack is
also required to hold local record or array data.
The stack is used by routines in user or supervisor state, that is, supervisor stack frames are
appended to user stack frames, and thus, parameters can be passed between user and
supervisor state. A small stack space must be reserved above UB. UB can then be set to a
higher value by the Frame Error handler to free stack space for error handling.