Imagecraft c compiler and development environment for the atmel avr

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ICCV8 for AVR – C Compiler for Atmel AVRAssembly Interface and Calling ConventionsExternal NamesExternal C names are prefixed with an underscore. For example, the function main is_main if referenced in an assembly module. Names are significant to 32 characters.To make an assembly object global, use two colons after the name. For example,_foo::.word 1.blkw 1(In the C file)extern int foo;Argument and Return RegistersIn the absence of a function prototype, integer arguments smaller than ints (forexample, char) must be promoted to int type. If the function prototype is available,the C standard leaves the decision to the compiler implementation. ICCV8 for AVRdoes not promote the argument types if the function prototype is available.If registers are used to pass byte arguments, it will use both registers but the higherregister is undefined. For example, if the first argument is a byte, both R16/R17 will beused with R17 being undefined. Byte arguments passed on the software stack alsotake up 2 bytes. We may change the behavior and pack byte arguments tightly insome future release.The first argument is passed in registers R16/R17 if it is an integer and R16/R17/R18/R19 if it is a long or floating point. The second argument is passed in R18/R19 ifavailable. All other remaining arguments are passed on the software stack. If R16/R17is used to pass the first argument and the second argument is a long or float, thelower half of the second argument is passed in R18/R19 and the upper half is passedon the software stack.Integer values are returned in R16/R17 and longs and floats are returned in R16/R17/R18/R19. Byte values are returned in R16 with R17 undefined.142
ICCV8 for AVR – C Compiler for Atmel AVRPreserved RegistersAn assembly function must save and restore the following registers. These registersare called preserved registers, since their contents are unchanged by a function call.Local variables are assigned to these registers by the compiler. R28/R29 or Y (this is the frame pointer) R10/R11/R12/R13/R14/R15/R20/R21/R22/R23You can ask the compiler not to use the registers R20, R21, R22, and R23, ; then youdo not need to save and restore these four registers. See Global RegistersVolatile RegistersThe other registers: R0/R1/R2/R3/R4/R5/R6/R7/R8/R9/R24/R25/R26/R27/R30/R31 SREGcan be used in a function without being saved or restored. These registers are calledvolatile registers, since their contents may be changed by a function call.Interrupt HandlersSince an interrupt handler operates asynchronously to the normal program operation,the interrupt handler or the functions it calls must not modify any machine registers.Therefore, an interrupt handler must save and restore all registers that it uses. This isdone automatically if you use the compiler capability to declare a C function as aninterrupt handler. If you write a handler in assembly and if it calls normal C functions,then the assembly handler must save and restore the volatile registers, since normalC functions do not preserve them.The exception is when you ask the compiler not to use the registers R20, R21, R22,and R23 ; then the interrupt handlers may use these four registers directly.StructurePassing by ValueIf passed by value, a structure is always passed through the stack, and not inregisters. Passing a structure by reference (i.e., passing the address of a structure) isthe same as passing the address of any data item; that is, a pointer to the structure(which is 2 bytes) is passed.143
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Imagecraft c compiler and development environment for the atmel avr

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