Imagecraft c compiler and development environment for the atmel avr

ICCV8 for AVR – C Compiler for Atmel AVRGlobal RegistersSometimes it is more efficient if your program has access to global registers. Forexample, in your interrupt handlers, you may want to increment a global variable thatanother part of the program may need to access. Using regular C global variables inthis manner may require more overhead than you want in interrupt handlers due to theoverhead of saving and restoring registers and the overhead of accessing memorywhere the global variables reside.You can ask the compiler not to use the registers R20, R21, R22, and R23 bychecking the Compiler->Options->Target->”Do Not Use R20..R23” option. You shouldnot check this option in general since the compiler may generate larger programbecause it has less registers to use. You cannot reserve other registers besides thisset.In rare cases when your program contains complex statements using long andfloating-point expressions, the compiler may complain that it cannot compile suchexpressions with this option selected. When that happens, you will need to simplifythose expressions.You can access these registers in your C program by using the pragma:#pragma global_register : :...for example:#pragma global_register timer_16:20 timer_8:22 timer2_8:23extern unsigned int timer_16;char timer_8, timer2_8;..#pragma interrupt_handler timer0:8 timer1:7void timer0(){timer_8++;}Note that you must still declare the datatypes of the global register variables. Theymust be of char, short, or int types, and you are responsible to ensure that theregister numbering is correct. A 2-byte global register will use the register number youspecified and the next register number to hold its content. For example, timer_16above is an unsigned int, and it will occupy register R20 and R21.Since these registers are in the upper 16 set of the AVR registers, very efficient codewill be generated for them when assigning constants, etc.123