In start.a51, there is defined a idata segment ?STACK.
?STACK SEGMENT IDATA
Now, in my asm file, I want to use the ?STACK to reset the SP,when using it directly as follows,
mov SP, #STACK-1
but there is an error: "error A45: UNDEFINED SYMBOL"
How can I do it, thanks.
For a very specific requirement, I once needed to do something similar.
The stack segment was specified as this:
?STACK Segment Idata Rseg ?STACK Stack: Ds 1 ;System stack area
With the start of stack labelled like this, I could make it public/extern and access it in a separate module.
So the code to initialise the stack was then this:
MOV SP,#Stack-1 ;Initialise 'real' stack
A very good idea, thanks very much. But there is still some trouble, as follows.
EXTRN IDATA (Stack) mov R0, #Stack;
The link error information is:
*** ERROR L127: UNRESOLVED EXTERNAL SYMBOL *** ERROR L128: REFERENCE MADE TO UNRESOLVED EXTERNAL
How could you make "Stack" public/extern, pls tell it in detail. Thanks.
The ISR may need a bit of stack space to save some registers. Preferably, the ISR should not make a lot of function calls requiring extra stack - most often, it shouldn't do any function calls at all.
Try to allocate static variables for the information needed by the ISR - and for any calls you just have to make from the ISR.
I recommend that you solve your stack usage problem, instead of continuing to play with the stack. Besides - playing with the stack means that the ISR doesn't need to know the start address of the main app stadck. It must save the stack pointer, before switching to a separate stack. After being done, this is the value that you must restore - not the start address of the stack.
How could you make "Stack" public/extern, pls tell it in detail
In the module that has the stack segment just include the line:
Public Stack
And in the module that needs to reference the stack segment do (as you have correctly done):
Extrn Idata (Stack)
Also, I notice your code snippet of:
mov R0, #Stack;
If you want to initialise the stack to the base of the block, you would normally use:
mov R0, #Stack-1
Because the stack pointer (on the 8051) increments before storing the data byte; sometimes referred to as a pre-increment style stack.
When adding "public Stack" in start.a51, all is ok. Thanks very much.