.nolist
; avr_registers.inc - my CPU register usage and naming for AVR
; author Neil Franklin, last modification 2008.10.22


; register usage and calling convention
;   callee-push/pop minimises code but allways runtime overhead
;   caller-push/pop minimises runtime overhead but repeated code
; large register sets like AVR allow splitting registers into 2 groups
;   some used for callee, caller pushes, only if it needs these
;   some used for caller, callee pushes, only if it needs these
;   with an good splitting policy only push/pop in multilevel calls


; instruction set specialty registers

; address registers for X Y and Z modes
; 16bit pairs, use *H:*L naming, T = Temporary

.def XL = R26   ; often 2 pointers quickly needed, not Y
.def XH = R27   ;  callee may use any time, caller push if still in use
.def YL = R28   ; only non-Z with +q addresseing, potential frame pointer
.def YH = R29   ;   must stay robust, callee push if used as third
.def ZL = R30   ; often special use, often changes, ideal for temp
.def ZH = R31   ;  callee may use any time, caller push if still in use

; other registers for 16bit arithmetic (adiw, sbiw)
; 16bit pair, also *H:*L naming, D = Double, A = Accumulator

.def AL = R24   ; never changed unneccessary, can have lasting stuff
.def AH = R25   ;   must survive entire caller, callee push if used

; registers for multiplication result
; 16bit pair, also *H:*L naming, M = Multiplication

.def ML = R0    ; change at any multiplication, not lasting stuff, also temp
.def MH = R1    ;   callee may use any time, caller push if still in use


; instruction set normal registers

; registers with immediate instructions (ldi, subi, sbci, cpi, andi, ori)
;   are rest of R16..R31, gives R16..R23

; immediate (loop) counters, C = count

.def C0 = R16   ; must survive entire loop, callee push if used
.def C1 = R17

; immediate accumulators for general computation, A = Accumulator

.def A0 = R18   ; must survive entire caller, callee push if used
.def A1 = R19

; immediate temporaries (and parameters) for general computation, T = Temp

.def T0 = R20   ; often changed, callee fast usage, caller push if still in use
.def T1 = R21

; immediate for interupts, I = Interrupt

.def I0 = R22   ; not used in normal code, only for interrupt, ISR without push
.def I1 = R23   ;   therefore also survive between multiple interrupts

; without immediate instructions
;   are rest of R0..R15, gives R2..R15

; no non-immediate (loop) counters, as usually loaded with ldi
;   preferably missuse an temp or push/pop if 3rd nested loop is needed

; non-immediate accumulators for general computation, A = Accumulator

.def A2 = R2    ; must survive entire caller, callee push if used
.def A3 = R3

; non-immediate temporaries (and parameters) for general computation, T = Temp

.def T2 = R4    ; often changed, callee fast usage, caller push if still in use
.def T3 = R5

; non-immediate for interupts, I = Interrupt

.def I2 = R6    ; not used in normal code, only for interrupt, without push
.def I3 = R7    ;   therefore also survive between multiple interrupts

; non-immediate fast global variables, G = Global
;   also (miss)use for other/special purposes, reserve specially

.def G0 = R8    ; must survive entire program, callee push if used
.def G1 = R9
.def G2 = R10
.def G3 = R11
.def G4 = R12
.def G5 = R13
.def G6 = R14
.def G7 = R15

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