/******************************************************************************

   The values which the registers (should) have upon exiting Shared_Setup():

   R0          no specific value; available to be used
   R1       has the value 0
   R3:R2    pointer to the BYTE count WORD before the data BYTEs
   R4-R10      no specific values; available to be used
   R11      3 MSBits: G,R,B enable; 1:Z, 4 LSBits: Port index   uVars.B[ n+1 ]
   R12      bPortPins, the output pins selected                 uVars.B[ n+2 ]
   R13      bIncrDecr, the increment or decrement amount        uVars.B[ n+3 ]
   R14      bMaxBright, the maximum color output value          uVars.B[ n+4 ]
   R15      uVars.B[] adder in the 328P; unused & available in 1284P & 2560
   R16         no specific value; available to be used
   R17      Mode Specific BYTE #1                               uVars.B[ n+7 ]
   R18      Mode Specific BYTE #2                               uVars.B[ n+8 ]
   R19      Mode Specific BYTE #3                               uVars.B[ n+9 ]
   R21:R20  number of data BYTES (pixels * 3) to write
   R22         scratch (values OR'd together)
   R23         no specific value; available to be used
   R25:R24  pointer to the HEAP; available to be used
   R27:R26  wLEDCount WORD pixel count                     uVars.B[ n+5, n+6 ]
   R29:R28  base SRAM address for uVars.B[ n ]
   R31:R30  pointer to the starting data location (= R3:R2 + 2)

      As an example, these 2 configurations:
   0200:  01 E2 1C FF FF 27 00 00-28 08 C8 00 05 00 FF FF  .....'..(.......
   0210:  00 E2 1C FF FF 28 00 00-08 28 C8 00 05 00 FF FF  .....(...(......

      Generate these register set upon entry, when called with R24 = 0:
 10   32   54   76   98  1110 1312 1514 1716 1918 2120 2322 2524 2726 2928 3130
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
0000 027E 1081 8128-0000 E210 FF1C 00FF-0000 2808 0078 0328-027E 0028 0200 0280
0000 027E 1081 8128-0000 E200 FF1C 00FF-0000 0828 0075 0327-027E 0027 0200 0280

   Passed: R3:R2 pointing to the BYTE count WORD before the data (as above)
           R29:R28 pointing to the base uVars.B[ n ] SRAM address (as above)

******************************************************************************/