HCS12 datasheet(427/452 Pages) FREESCALE

427 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

427

C.3.2 Allocating and Deallocating Stack Space

The LEAS instruction can be used to allocate or deallocate space on the

stack for temporary variables:

LEAS

–10,S

;Allocate space for 5 16-bit integers

LEAS

10,S

;Deallocate space for 5 16-bit ints

The (de)allocation can even be combined with a register push or pull as in

this example:

LDX

8,S+

;Load return value and deallocate

X is loaded with the 16-bit integer value at the top of the stack, and the stack

pointer is adjusted up by eight to deallocate space for eight bytes worth of

temporary storage. Post-increment indexed addressing is used in this

example, but all four combinations of pre/post increment/decrement are

available (offsets from –8 to +8 inclusive, from X, Y, or SP). This form of

indexing can often be used to get an index (or stack pointer) adjustment for

free during an indexed operation (the instruction requires no more code

space or cycles than a zero-offset indexed instruction).

C.3.3 Frame Pointer

In the C language, it is common to have a frame pointer in addition to the

CPU stack pointer. The frame is an area of memory within the system stack

which is used for parameters and local storage of variables used within a

function subroutine. The following is a description of how a frame pointer

can be set up and used.

First, parameters (typically values in CPU registers) are pushed onto the

system stack prior to using a JSR or CALL to get to the function subroutine.

At the beginning of the called subroutine, the frame pointer of the calling

program is pushed onto the stack. Typically, an index register, such as X, is

used as the frame pointer, so a PSHX instruction would save the frame

pointer from the calling program.

Next, the called subroutine establishes a new frame pointer by executing a

TFR S,X. Space is allocated for local variables by executing an

LEAS –n,S, where n is the number of bytes needed for local variables.

Notice that parameters are at positive offsets from the frame pointer while

locals are at negative offsets. In the M68HC11, the indexed addressing

mode uses only positive offsets, so the frame pointer always points to the

lowest address of any parameter or local. After the function subroutine

finishes, calculations are required to restore the stack pointer to the

HCS12 Datasheet(PDF) 427 Page - Freescale Semiconductor, Inc

HCS12 Datasheet(HTML) 427 Page - Freescale Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL