GMS30C2116 Datasheet, PDF (43/322 Pages) Hynix Semiconductor – USERS MANUAL

ARCHITECTURE
1-23
1.7 Stack (continued)
A currently activated function A has a frame length of FL = 13, FL = 3(required to save
passed parameters) + 10(received). Registers L0..L6 are to be retained through a
subsequent call, registers L7..L12 are temporaries. A call to function B needs 2 parameters
to be passed. The parameters are placed by function A in registers L7 and L8 before calling
B. The Call instruction addresses L9 as destination for the return PC and return SR register
pair to be used by function B on return to function A.
On entry of function B, the new frame of B has an implicit length of FL = 6. It starts
physically at the former register L9 of frame A. However, since the frame pointer FP has
been incremented by 9 by the Call instruction, this register location is now being addressed
as L0 of frame B. The passed parameters cannot be addressed because they are located
below the new register L0 of frame B. To make them addressable, a Frame instruction
decrements the frame pointer FP by 2. Then, parameter 1 and 2 passed to B can be
addressed as registers L0 and L1 respectively. Note that the return PC is now to be
addressed as L2!
The Frame instruction in B specifies also the new, complete frame length FL = 11
(including the passed parameters as well as the return PC and return SR pair). Besides, a
new reserve of 10 registers for subsequent function calls and traps is provided in the
register stack. A possible overflow of the register stack is checked and handled
automatically by the Frame instruction. A program needs not and must not pay attention to
register stack overflow.
At the end of function B, a Return instruction returns control to function A and restores the
frame A. A possible underflow of the register stack is handled also automatically; thus, the
frame A is always completely restored, regardless whether it was wholly or partly pushed
into the memory part of the stack before (in the case when B called other functions).
In the present example with the frame length of FL = 13, any suitable destination register
up to L13 could be specified in the Call instruction. The parameters to be passed to the
function B would then be placed in L11 and L12. It is even possible to append a new frame
to a frame with a length of FL = 16 (coded as FL = 0 in the status register SR): the
destination register in the Call instruction is then coded as L0, but interpreted as the
register past L15.
See also sections 3.27. Call instruction, 3.29. Frame instruction and 3.30. Return
instruction for further details.
Note: With an average frame length of 8 registers, ca. 7..8 Frame instructions succeed a
pulling Return instruction until a push occurs and 7..8 Return instructions succeed a
pushing Frame instruction until a pull occurs. Thus, the built-in hysteresis makes pushing
and pulling a rare event in regular programs!
Figure 1.14 represents the stack frame pushing and popping. When the register part of the
stack A and X overlapped modulo 64 (the register part of stack was full), the frame
instruction for frame X pushed the number of words in frame A to the memory part of the
stack according to the space required for frame X. When the process returned to frame A,
the return instruction pulled the number of words form the memory part of the stack to the
register part of the stack.