Information about the sdcc port

1.  Make sure you have something newer than sdcc 2.7.2 (I used
    2.7.3).  do "sdcc --version" to check.

2.  The sdcc compiler is rather touchy so make small changes and then
    test.

3.  Using lots of printf's when DEBUG is defined can cause stack
    problems so use them sparingly.  fprintf uses lots of stack so I
    had to tweak sd18f2550.lkr to allow the stack to grow down to
    0x1f0 from 0x2ff by not allowing 0x1f0 to 0x1ff to be used:

DATABANK   NAME=gpr1       START=0x100          END=0x1F0

4.  Stay away from (void *) pointers, they will end up causing things
    like:

 ;	.line	368; system/usb/usbctrltrf/usbctrltrf.c	pSrc = (void *)&bdt_data.CtrlTrfData;
0038f6   0e28     movlw	0x28               	MOVLW	0x28
0038f8   0000     nop                      	BANKSEL	LOW(_bdt_data)
0038fa   2500     addwf	0, 0, 0x1          	ADDWF	LOW(_bdt_data), W, B
0038fc   0101     movlb	0x1                	BANKSEL	_pSrc
0038fe   6ff8     movwf	0xf8, 0x1          	MOVWF	_pSrc, B
003900   0e00     movlw	0                  	MOVLW	0x00
003902   0004     clrwdt                   	BANKSEL	HIGH(_bdt_data)

  see the clrwdt instruction?  That's a "clear watchdog timer"
instruction, clearly not right.

5.  The original Microchip code had a POINTER struct that caused sdcc
    to go crazy, like this:

	.line	75; main.c	*pDst.bRam = *pSrc.bRom;
000f48   c093     movff	0x93, 0x2          	MOVFF	_pDst, r0x02
000f4a   f002
000f4c   c094     movff	0x94, 0x3          	MOVFF	(_pDst + 1), r0x03
000f4e   f003
000f50   c095     movff	0x95, 0x4          	MOVFF	(_pDst + 2), r0x04
000f52   f004
000f54   0e90     movlw	0x90               	MOVLW	_pSrc
000f56   6ef6     movwf	0xf6, 0            	MOVWF	TBLPTRL
000f58   0e91     movlw	0x91               	MOVLW	(_pSrc + 1)
000f5a   6ef7     movwf	0xf7, 0            	MOVWF	TBLPTRH
000f5c   0e92     movlw	0x92               	MOVLW	(_pSrc + 2)
000f5e   6ef8     movwf	0xf8, 0            	MOVWF	TBLPTRU

  Here it's loading up TBLPTR with the address 0x929190, not quite
right!

6. Using "data" as a passed parameter is a problem, not sure why.
   It's been changed to "d".


7. The sdcc compiler needs to
have the pragmas with at least one use.  In other words you cant do:

----------------------------------
file A:

#pragma udata usbram5 cdc_notice
(no usage of cdc_notice)

file B:

cdc_notice = 0;
----------------------------------

  You have to move the pragma to file B:

----------------------------------
file A:

(no usage of cdc_notice)

file B:

#pragma udata usbram5 cdc_notice
cdc_notice = 0;
----------------------------------