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; avr_registers.inc - my CPU register usage and naming for AVR
; author Neil Franklin, last modification 2008.11.16


; 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, 16bit

; registers for 16bit X Y and Z addressing
;   on all 16bit pairs use *H:*L naming

.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

.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 XL = R26   ; often 2 pointers are quickly needed, not Y for this
.def XH = R27   ;  callee may use any time, caller push if still in use

; other registers for 16bit arithmetic (adiw, sbiw)
;   on all 16bit pairs use *H:*L naming, D = Double

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

; registers for 16bit multiplication result
;   on all 16bit pairs use *H:*L naming, M = Multiplication

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


; instruction set normal registers, 8bit

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

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

.def T0 = R16   ; often changed, callee fast usage, caller push if still in use
.def T1 = R17
.def T2 = R18

; immediate saved (and (loop) counters), S = Save

.def S0 = R19   ; must survive entire caller, incl calls, callee push if used
.def S1 = R20   ;   are used longer, may be renamed to make code more readable
.def S2 = R21

; immediate for interupts service routines, 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


; instruction set reduced functionality registers, 8bit

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

; use all as fast direct access (register not memory) variables, V = Variable

.def V0 = R2    ; must survive entire program, callee push if used
.def V1 = R3
.def V2 = R4
.def V3 = R5
.def V4 = R6
.def V5 = R7
.def V6 = R8
.def V7 = R9
.def V8 = R10
.def V9 = R11
.def V10 = R12
.def V11 = R13
.def V12 = R14
.def V13 = R15

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