Call C function (that calls another C function) from assembly code

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Read-Only Author Nguyen Anh Huy Posted 8-Feb-2006 03:41 GMT Toolset C51	Call C function (that calls another C function) from assembly code Nguyen Anh Huy 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.
Read-Only Author Andy Neil Posted 8-Feb-2006 06:51 GMT Toolset C51	It's irrelevant! Andy Neil "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?!
Read-Only Author Reinhard Keil Posted 8-Feb-2006 08:17 GMT Toolset C51	RE: It's irrelevant! Reinhard Keil There is extensive information about this on this web side. For example: http://www.keil.com/support/docs/2163.htm Reinhard
Read-Only Author erik malund Posted 8-Feb-2006 14:41 GMT Toolset C51	RE: It's irrelevant! erik malund 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
Read-Only Author Andy Neil Posted 8-Feb-2006 15:16 GMT Toolset C51	Registers after call Andy Neil "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
Read-Only Author erik malund Posted 8-Feb-2006 15:34 GMT Toolset C51	RE: Registers after call erik malund 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
Read-Only Author Nguyen Anh Huy Posted 9-Feb-2006 01:58 GMT Toolset C51	I have read the manual Nguyen Anh Huy 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 I think it must be: MOV DPTR, #_?function?BYTE+3 Does it right?
Read-Only Author Andy Neil Posted 9-Feb-2006 07:05 GMT Toolset C51	Black box. Andy Neil "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.
Read-Only Author Andy Neil Posted 9-Feb-2006 07:58 GMT Toolset C51	LARGE Model example Andy Neil "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

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