Imagecraft c compiler and development environment for the atmel avr

ICCV8 for AVR – C Compiler for Atmel AVRAccessing Memory Outside of the 64K RangeWith 16-bit pointers, the AVR can normally access 64K bytes of data memory, 64Kwords of program memory used for functions and 64K bytes of program memory forconstant data. The latter requires a clarification: since code must be aligned on a wordboundary, the AVR uses word addresses to access code (e.g., for a function call). Thisallows a function call to reach the full 128K byte / 64K word of the Mega128 flashmemory space with a 16-bit word address. However, constant data in the flashmemory uses byte addresses, so the AVR normally can only access constant data inthe lower 64K bytes of the flash memory using the LPM instruction.To get around these limitations, the AVR uses the RAMPZ IO register to allow greaterthan-64Kaccess to flash memory. The XMega has four such IO registers: RAMPX,RAMPY, RAMPZ, and RAMPD, to allow greater-than-64K access to both data andflash memory.Currently we do not support automatic changing of the RAMP? IO registers topreserve the simplicity of using 16-bit pointers. Allowing a mix of 16- and 24/32-bitpointers would increase the code size significantly, and it is more efficient if youmanage the paging yourselves using the following rules and library functions.Non-XMega: Managing the RAMPZ RegisterThe non-XMega AVR uses the RAMPZ to access constant data above the 64K-byteboundary (using the elpm instruction) and does not define the other RAMP? registers.Therefore it is easiest if you just change RAMPZ as needed when accessing aconstant item. With careful managment (see Managing Constant Data), you can setand reset RAMPZ as needed.The rest of this manual section is for XMega only, as the topics do not apply to non-XMega parts.XMega: Managing the RAMP? RegistersThe XMega uses all four RAMP? registers for accessing data memory, and RAMPZ isalso used for accessing constant data above 64K bytes. We recommend thatgenerally to leave the RAMP? registers at their values initialized in the Startup File,which is zero in all cases except for RAMPZ. RAMPZ is set to zero unless you arebuilding a bootloader project that is located above the 64K-byte boundary, in whichcase, it is set to 1, 2, or 3 depending on the location of the bootloader. These defaultswork well most of the time as most data and constant data reside in their respectivelower 64K-byte areas.127