Download User Manual ST10F272

UM0381
System programming
The effect of the address transformation is that the physical stack addresses wrap around
from the end of the defined area to its beginning. When flushing and filling the internal stack,
this circular stack mechanism only requires to move that portion of stack data which is really
to be re-used (the upper part of the defined stack area) instead of the whole stack area.
Stack data that remain in the lower part of the internal stack need not be moved by the
distance of the space being flushed or filled, as the stack pointer automatically wraps
around to the beginning of the freed part of the stack area.
Note:
This circular stack technique is applicable for stack sizes of 32 to 512 words (STKSZ =
‘000b’ to ‘100b’), it does not work with option STKSZ = ‘111b’, which uses the complete
IRAM for system stack.
In the latter case the address transformation mechanism is deactivated.
When a boundary is reached, the stack underflow or overflow trap is entered, where the
user moves a predetermined portion of the internal stack to or from the external stack. The
amount of data transferred is determined by the average stack space required by routines
and the frequency of calls, traps, interrupts and returns. In most cases this will be
approximately one quarter to one tenth the size of the internal stack. Once the transfer is
complete, the boundary pointers are updated to reflect the newly allocated space on the
internal stack. Thus, the user is free to write code without concern for the internal stack
limits. Only the execution time required by the trap routines affects user programs.
The following procedure initializes the controller for usage of the circular stack mechanism:
●
Specify the size of the physical system stack area within the IRAM (bit-field STKSZ in
register SYSCON).
●
Define two pointers, which specify the upper and lower boundary of the external stack.
These values are then tested in the stack underflow and overflow trap routines when
moving data.
●
Set the stack overflow pointer (STKOV) to the limit of the defined internal stack area
plus six words (for the reserved space to store two interrupt entries).
The internal stack will now fill until the overflow pointer is reached. After entry into the
overflow trap procedure, the top of the stack will be copied to the external memory. The
internal pointers will then be modified to reflect the newly allocated space. After exiting from
the trap procedure, the internal stack will wrap around to the top of the internal stack, and
continue to grow until the new value of the stack overflow pointer is reached.
When the underflow pointer is reached while the stack is emptied the bottom of stack is
reloaded from the external memory and the internal pointers are adjusted accordingly.
Linear stack
The ST10F272 also offers a linear stack option (STKSZ = ‘111b’), where the system stack
may use the complete IRAM area. This provides a large system stack without requiring
procedures to handle data transfers for a circular stack. However, this method also leaves
less RAM space for variables or code. The RAM area that may effectively be consumed by
the system stack is defined via the STKUN and STKOV pointers. The underflow and
overflow traps in this case serve for fatal error detection only. For the linear stack option all
modifiable bit of register SP are used to access the physical stack. Although the stack
pointer may cover addresses from 00’F000h up to 00’FFFEh the (physical) system stack
must be located within the IRAM and therefore may only use the address range 00’F600h to
00’FDFEh. It is the user’s responsibility to restrict the system stack to the IRAM range.
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