Hi all, I want to call a C function (for example: func1() ) from my old assembly code. The problem is that if func1() calls another C function( func2() ), what will happen? How does the compiler pass parameters to the second function? If it also passes through R1-R7, does it destroy the parameters of func1()? Thanks for reading this topic.
"The problem is that if func1() calls another C function( func2() )" That is entirely irrelevant! The C51 calling convention defines how to call a function. It is the same for every function - that's the whole point of a calling convention. "If it also passes through R1-R7, does it destroy the parameters of func1()?" It will ensure that it does not destroy anything that it'll need again! Note that your assembler code must assume that the C51 function call will destroy all registers. You have read the chapter in the Manual on interfacing C51 and Assembler, haven't you?!
There is extensive information about this on this web side. For example: http://www.keil.com/support/docs/2163.htm Reinhard
one thing not mentioned in the above link: C is free to use (and not save) ANY register it see fit. This may/will change with different releases and different optimizations. Thus so called "testting" to find which registers are affecter is worthless as "testing' often is. Thus you MUST code like this lcall Cfunct ; ALL REGISTRES INVALID AFTER CALL Erik
"Thus you MUST code like this lcall Cfunct ; ALL REGISTRES INVALID AFTER CALL" If the function returns a value, it will use registers as defined here: http://www.keil.com/support/man/docs/c51/c51_ap_funcret.htm
correct, but that register is still "destroyed" I once had to fix somebodys code that called C from assembly and, the C called only modified was it r4? under one particular condition and, of course you had the "monthly intermittent". So much about mixing C and assembler is done with assumptions and thus likely causes of "unexplained failure" Erik
To Andy Neil, I have read the manual, of course. But in the manual's example, it only calls one C function, and it doesn't save any registers!!! So I confuse that if the first C function calls another C function in it, does it save the registers? I also see that, in the Large Model Example, page 171, line 11:
MOV R0, #?_funtion?BYTE+3
MOV DPTR, #_?function?BYTE+3
"I confuse that if the first C function calls another C function" What happens inside the function is entirely irrelevant - See my original post. You just need to see the function as a "black box" - it has a defined interface, and what actually goes on inside the box is of no concern.
"I also see that, in the Large Model Example, page 171, line 11: MOV R0, #?_funtion?BYTE+3 I think it must be: MOV DPTR, #_?function?BYTE+3" Hmmm... I think you're right. Looks like a cut-and-paste error - they've copied the COMPACT example, and forgotten to change R0 to DPTR? You could check this by compiling an example yourself... also here: http://www.keil.com/support/man/docs/c51/c51_ap_largemodel.htm