DCD equivalent in SX/B or a way to access each bit in a loop?

basicstampede

In BS2, there is a command DCD which returns the highest bit position of 1 in a Byte.

I don't think there is a equivalent in SX/B, but what is a clever way of doing this?

e.g. for %00101111, I want to output %00100000

I tried the below code:

a VAR Byte
b VAR Byte
idx VAR Byte

Main:
··a = %01110000
· FOR idx = 7 to 0
··· b = a.idx
··· LEDs = b
· NEXT·
· GOTO Main

The above code will not compile because of a.idx is invalid.·
Is there a way to sequentially access each bit of a byte variable?
·

JonnyMac

Here's code for NCD (which is what you want) and DCD:

NCD             FUNC    1, 1, 2
DCD             FUNC    2, 1

Edit: 12/24/2008: I had a project that required an NCD function so I went back a looked at my code again -- here are better versions of both.

' Use: result = NCD value
' -- returns position of highest "1" in value
' -- 0 (none) to 16 (bit15)

FUNC NCD
  ASM
    SB    __PARAMCNT.1                          ' skip if word
     CLR  __WPARAM12_MSB                        ' clear MSB if byte
    MOV   __PARAM3, #16                         ' initialize position

NCD_Loop:
    JB    __WPARAM12_MSB.7, @NCD_Exit           ' exit if bit is "1"
    CLC                                         '  else shift left
    RL    __WPARAM12_LSB
    RL    __WPARAM12_MSB
    DEC   __PARAM3                              '  and update position
    JNZ   @NCD_Loop                             '  if pos = 0, abort

NCD_Exit:
    MOV   __PARAM1, __PARAM3                    ' return position
  ENDASM
  ENDFUNC

' -------------------------------------------------------------------------

' Use: result = DCD bitPos
' -- returns 16-bit value based on bitPos (1 to 16)
' -- returns 0 if value illegal

FUNC DCD
  ASM
    MOV   __PARAM3, __PARAM1                    ' copy position
    CLR   __WPARAM12_LSB                        ' clear result
    CLR   __WPARAM12_MSB
    DEC   __PARAM3                              ' zero-adjust position
    CJA   __PARAM3, #15, @DCD_Exit              ' abort if illegal
    INC   __WPARAM12_LSB                        ' set bit 0
    TEST  __PARAM3                              ' check for pos 0
    JZ    @DCD_Exit                             '  abort if 0

DCD_Loop:
    CLC                                         ' zero-pad for shift
    RL    __WPARAM12_LSB                        ' shift result
    RL    __WPARAM12_MSB
    DJNZ  __PARAM3, @DCD_Loop                   ' update pos

DCD_Exit:
  ENDASM
  ENDFUNC

Post Edited (JonnyMac) : 12/24/2008 11:32:13 PM GMT