Imagecraft c compiler and development environment for the atmel avr

ICCV8 for AVR – C Compiler for Atmel AVRSpecific AVR IssuesThere are a wide array of AVR devices available from Atmel, differing in memory sizesand peripherals. This section describes some of the AVR and device specific issues.AVR FeaturesAVR uses the Harvard architecture and separate the program and data memory.See Program Data and Constant Memory.Program memory size is specified in bytes (e.g. M32 has 32K bytes of flash) buteach instruction is at least 2 bytes long. Therefore, program locations encoded inthe instructions are word addresses. When used as storage for literal tables etc.,the program memory is byte addressable using the lpm/elpm family ofinstructions.ICCV8 for AVR programs such as the compiler proper, the assembler and linkeruse byte addresses in most usages and the tools do the appropriate word addressconversion as necessary.The registers are not all general purpose. AVR instructions that use immediatesource operands (e.g. addi, ldi etc.) only operate on the upper 16 registersR16 to R31. There are only 3 set of “pointer” registers: R26/R27 or X, R28/R29 orY, and R30/R31 or Z. Y and Z can use the displacement mode with an offset but Xcannot use such mode.The generated code use two stacks (see Program Data and Constant Memory),the native hardware stack for CALL return addresses and a software stack usingthe Y pointer. While this makes it more challenging to ensure that both stackshave sufficient space, the alternative of using a single stack generatessubstantially more code.Device-Specific InstructionsDevices with 8K or less bytes of program memory do not support the jmp and callinstructions, and only the rjmp and rcall relative instructions. The initial generationof the AVR (sometimes known as “classic” AVRs) do not have the newer instructionssuch as the multiply instructions and the extended lpm instructions.139