AVR R Register Usage

Overview

GCBASIC uses a set of system variables for internal calculations, temporary storage, division/multiplication results, delays, string handling, and more. These are mapped to General Purpose Registers (GPRs) on AVR targets to avoid RAM access overhead where possible.

The mapping is determined by the GCBASIC source code function GetRegisterLoc(RegName As String), which returns the register number (0–31) or -1 if not found (falling back to RAM).

Key points:

On chips with only 16 GPRs (ChipGPR = 16, e.g., ATtiny4/5/9/10 — physically mapped as R16–R31), system variables are placed in R16–R31.
On chips with full 32 GPRs (ChipGPR != 16, most AVRs), system variables use a mix of low (R0–R5) and high (R21–R31) registers.
R6–R20 are typically unused for system variables in full mode (available for user code).
X, Y, Z pointers (R26:R27, R28:R29, R30:R31) are not allocated as system variables — reserved for addressing.
If DestLoc = -1 (e.g., SysSignByte on 16-GPR chips), the variable uses SRAM (RAM) instead of GPR.

GPR16 Allocation (ChipGPR = 16)

Allocation for low-GPR chips (only R16–R31 available).

Register	Primary System Variable(s)	Aliases / Shared With	Notes
R16	SysCalcTempX, SysByteTempX, SysWordTempX, SysIntegerTempX, SysLongTempX	-	Multi-precision calculation temp (low byte)
R17	SysCalcTempX_H, SysWordTempX_H, SysIntegerTempX_H, SysLongTempX_H	-	High byte of above
R18	SysCalcTempX_U, SysLongTempX_U, SysDivMultX	-	Upper byte (for 32-bit ops) or division/multiplication temp
R19	SysCalcTempX_E, SysLongTempX_E, SysDivMultX_H	-	Extra/remainder byte
R20	SysDivLoop, SysBitTest	-	Division loop counter or bit test temp
R21	SysValueCopy	-	Temporary copy for value transfers (e.g. stack/out ops)
R22	SysCalcTempA, SysByteTempA, SysWordTempA, SysIntegerTempA, SysLongTempA	-	Primary calculation temp A (low)
R23	SysCalcTempA_H, SysWordTempA_H, SysIntegerTempA_H, SysLongTempA_H	-	High byte
R24	SysCalcTempA_U, SysLongTempA_U, SysDivMultA	-	Upper byte
R25	DelayTemp, SysCalcTempA_E, SysLongTempA_E, SysDivMultA_H, SysStringLength	DelayTemp2 (partial)	Delay counters + string length
R26	DelayTemp2, SysStringA	-	Secondary delay or string pointer low
R27	SysWaitTempUS, SysWaitTemp10US, SysWaitTempM, SysStringA_H	-	Microsecond delay temps + string high
R28	SysWaitTempUS_H, SysWaitTempH, SysCalcTempB, SysByteTempB, SysWordTempB, SysIntegerTempB, SysLongTempB, SysStringB	-	Byte/word/long temp B (low) + millisecond delay high
R29	SysWaitTempMS, SysCalcTempB_H, SysWordTempB_H, SysIntegerTempB_H, SysLongTempB_H, SysStringB_H	-	High bytes
R30	SysWaitTempMS_H, SysCalcTempB_U, SysLongTempB_U, SysDivMultB, SysReadA	-	Upper bytes or read temp
R31	SysWaitTemp10MS, SysWaitTemps, SysCalcTempB_E, SysLongTempB_E, SysDivMultB_H, SysReadA_H	-	Extra temps / high bytes

Full Register Set Allocation (ChipGPR != 16)

Allocation for full 32-GPR chips (R0–R31 available).

Register	Primary / Common System Variable(s)	When ChipGPR=16 (shifted to)	Notes / Conflicts
R0	SysCalcTempX, SysByteTempX, SysWordTempX, SysIntegerTempX, SysLongTempX	R16	Low byte of multi-byte calc temp X
R1	SysCalcTempX_H, SysWordTempX_H, SysIntegerTempX_H, SysLongTempX_H	R17	High byte
R2	SysCalcTempX_U, SysLongTempX_U, SysDivMultX	R18	Upper / division temp
R3	SysCalcTempX_E, SysLongTempX_E, SysDivMultX_H	R19	Extra byte
R4	SysSignByte	RAM	Sign byte for signed operations
R5	SysDivLoop, SysBitTest	R20	Loop / bit temp
R6–R20	(unused for system vars)	-	Available for user variables or scratch
R21	SysValueCopy	R21	Value copy register (used in stack init, etc.)
R22	SysCalcTempA, SysByteTempA, SysWordTempA, SysIntegerTempA, SysLongTempA	R22	Primary calculation temp A (low)
R23	SysCalcTempA_H, SysWordTempA_H, SysIntegerTempA_H, SysLongTempA_H	R23	High byte
R24	SysCalcTempA_U, SysLongTempA_U, SysDivMultA	R24	Upper byte
R25	DelayTemp, SysCalcTempA_E, SysLongTempA_E, SysDivMultA_H, SysStringLength	R25	Delay + string length
R26	DelayTemp2, SysStringA	R26	Secondary delay / string low
R27	SysWaitTempUS etc., SysStringA_H	R27	Microsecond delays + string high
R28	SysWaitTempUS_H, SysWaitTempH, SysCalcTempB etc.	R28	Temp B low + delays
R29	SysWaitTempMS, SysCalcTempB_H etc.	R29	High bytes
R30	SysWaitTempMS_H, SysCalcTempB_U etc.	R30	Upper bytes
R31	SysWaitTemp10MS, SysWaitTemps, SysCalcTempB_E etc.	R31	Extra temps
R26:R27 (X)	Not used as system vars	-	Reserved for pointer operations
R28:R29 (Y)	Not used as system vars	-	Reserved for pointer / stack frame
R30:R31 (Z)	Not used as system vars	-	Reserved for program memory / indirect jumps

Alphabetical Variable Mapping for GPR16 Mode (ChipGPR = 16)

Variables sorted alphabetically, with their assigned register (or RAM if -1).

Variable Name	Register	Notes
delaytemp	R25	Delay counter
delaytemp2	R26	Secondary delay counter
sysbittest	R20	Bit test temp
sysbytetempa	R22	Byte temp A
sysbytetempb	R28	Byte temp B
sysbytetempx	R16	Byte temp X
syscalctempa	R22	Calc temp A (low)
syscalctempa_e	R25	Extra byte A
syscalctempa_h	R23	High byte A
syscalctempa_u	R24	Upper byte A
syscalctempb	R28	Calc temp B (low)
syscalctempb_e	R31	Extra byte B
syscalctempb_h	R29	High byte B
syscalctempb_u	R30	Upper byte B
syscalctempx	R16	Calc temp X (low)
syscalctempx_e	R19	Extra byte X
syscalctempx_h	R17	High byte X
syscalctempx_u	R18	Upper byte X
sysdivloop	R20	Division loop counter
sysdivmulta	R24	Division/multiplication temp A
sysdivmulta_h	R25	High byte
sysdivmultb	R30	Division/multiplication temp B
sysdivmultb_h	R31	High byte
sysdivmultx	R18	Division/multiplication temp X
sysdivmultx_h	R19	High byte
sysintegertempa	R22	Integer temp A (low)
sysintegertempa_h	R23	High byte
sysintegertempb	R28	Integer temp B (low)
sysintegertempb_h	R29	High byte
sysintegertempx	R16	Integer temp X (low)
sysintegertempx_h	R17	High byte
syslongtempa	R22	Long temp A (low)
syslongtempa_e	R25	Extra byte
syslongtempa_h	R23	High byte
syslongtempa_u	R24	Upper byte
syslongtempb	R28	Long temp B (low)
syslongtempb_e	R31	Extra byte
syslongtempb_h	R29	High byte
syslongtempb_u	R30	Upper byte
syslongtempx	R16	Long temp X (low)
syslongtempx_e	R19	Extra byte
syslongtempx_h	R17	High byte
syslongtempx_u	R18	Upper byte
sysreada	R30	Read temp A
sysreada_h	R31	High byte
syssignbyte	RAM	Sign byte (uses RAM on 16-GPR chips)
sysstringa	R26	String pointer A (low)
sysstringa_h	R27	High byte
sysstringb	R28	String pointer B (low)
sysstringb_h	R29	High byte
sysstringlength	R25	String length
sysvaluecopy	R21	Value copy temp
syswaittemp10ms	R31	10ms wait temp
syswaittemp10us	R27	10us wait temp
syswaittemph	R28	Hour wait temp
syswaittempm	R27	Minute wait temp
syswaittempms	R29	ms wait temp
syswaittempms_h	R30	High byte
syswaittemps	R31	Second wait temp
syswaittempus	R27	us wait temp
syswaittempus_h	R28	High byte
syswordtempa	R22	Word temp A (low)
syswordtempa_h	R23	High byte
syswordtempb	R28	Word temp B (low)
syswordtempb_h	R29	High byte
syswordtempx	R16	Word temp X (low)
syswordtempx_h	R17	High byte

Alphabetical Variable Mapping for Full Mode (ChipGPR != 16)

Variables sorted alphabetically, with their assigned register.

Variable Name	Register	Notes
delaytemp	R25	Delay counter
delaytemp2	R26	Secondary delay counter
sysbittest	R5	Bit test temp
sysbytetempa	R22	Byte temp A
sysbytetempb	R28	Byte temp B
sysbytetempx	R0	Byte temp X
syscalctempa	R22	Calc temp A (low)
syscalctempa_e	R25	Extra byte A
syscalctempa_h	R23	High byte A
syscalctempa_u	R24	Upper byte A
syscalctempb	R28	Calc temp B (low)
syscalctempb_e	R31	Extra byte B
syscalctempb_h	R29	High byte B
syscalctempb_u	R30	Upper byte B
syscalctempx	R0	Calc temp X (low)
syscalctempx_e	R3	Extra byte X
syscalctempx_h	R1	High byte X
syscalctempx_u	R2	Upper byte X
sysdivloop	R5	Division loop counter
sysdivmulta	R24	Division/multiplication temp A
sysdivmulta_h	R25	High byte
sysdivmultb	R30	Division/multiplication temp B
sysdivmultb_h	R31	High byte
sysdivmultx	R2	Division/multiplication temp X
sysdivmultx_h	R3	High byte
sysintegertempa	R22	Integer temp A (low)
sysintegertempa_h	R23	High byte
sysintegertempb	R28	Integer temp B (low)
sysintegertempb_h	R29	High byte
sysintegertempx	R0	Integer temp X (low)
sysintegertempx_h	R1	High byte
syslongtempa	R22	Long temp A (low)
syslongtempa_e	R25	Extra byte
syslongtempa_h	R23	High byte
syslongtempa_u	R24	Upper byte
syslongtempb	R28	Long temp B (low)
syslongtempb_e	R31	Extra byte
syslongtempb_h	R29	High byte
syslongtempb_u	R30	Upper byte
syslongtempx	R0	Long temp X (low)
syslongtempx_e	R3	Extra byte
syslongtempx_h	R1	High byte
syslongtempx_u	R2	Upper byte
sysreada	R30	Read temp A
sysreada_h	R31	High byte
syssignbyte	R4	Sign byte
sysstringa	R26	String pointer A (low)
sysstringa_h	R27	High byte
sysstringb	R28	String pointer B (low)
sysstringb_h	R29	High byte
sysstringlength	R25	String length
sysvaluecopy	R21	Value copy temp
syswaittemp10ms	R31	10ms wait temp
syswaittemp10us	R27	10us wait temp
syswaittemph	R28	Hour wait temp
syswaittempm	R27	Minute wait temp
syswaittempms	R29	ms wait temp
syswaittempms_h	R30	High byte
syswaittemps	R31	Second wait temp
syswaittempus	R27	us wait temp
syswaittempus_h	R28	High byte
syswordtempa	R22	Word temp A (low)
syswordtempa_h	R23	High byte
syswordtempb	R28	Word temp B (low)
syswordtempb_h	R29	High byte
syswordtempx	R0	Word temp X (low)
syswordtempx_h	R1	High byte

Additional Notes

SysValueCopy (R21) — Frequently used for temporary value transfers, especially during stack pointer setup (out SPH/SPL) or register copies.
Delay registers — DelayTemp, DelayTemp2, and wait temps (SysWaitTempUS, SysWaitTempMS, etc.) are reused across delay routines.
Division/Multiplication — Remainder and intermediate results use SysDivMultX, SysDivMultA, etc.
String handling — SysStringA, SysStringB, SysStringLength for string operations.
No overlap with X/Y/Z — GCBASIC preserves these for ld, st, lpm, elpm, ijmp, etc.
UserCodeOnlyEnabled mode — May reduce usage of some temps if init code is skipped, but mappings remain the same.
ChipGPR detection — Determined from chip definition files; affects only low-GPR chips (e.g., ATtiny4/5/9/10 series).

This mapping is derived directly from the GetRegisterLoc function in GCBASIC source code.

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