avr_r_register_usage - GreatCowBASIC/Help GitHub Wiki
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,SysStringLengthfor 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.