In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Jon Williams wrote:

Here's what I would do to create a 20-byte column buffer:

colBufLo   VAR    Byte (16)        ' define 20-byte array
colBufHi   VAR    Byte (4)

I would also define a separate column and buffer index pointers:

colPntr    VAR    Byte
bufIdx     VAR    Byte

Now the ISR is a can handle the code pretty cleanly:

' ---------------------------
  INTERRUPT
' ---------------------------

ISR_Start:
  LEDs = %00000000                 ' blank outputs to prevent ghosting
  INC colPntr                      ' update column pointer
  IF colPntr = 20 THEN             ' roll-over if at end
    colPntr = 0
  ENDIF
  IF colPntr.4 = 0 THEN            ' in low portion of buffer? 
    idx = colPntr & %00001111      ' calculate buffer index
    LEDs = colBufLo(bufIdx)        ' output current column
  ELSE
    idx = colPntr & %00000011
    LEDs = colBufHi(bufIdx)
  ENDIF

ISR_Exit:
  RETURNINT 63

Note that if you use a 32-byte buffer then you can simplify the buffer pointer manipulation (with roll-over) like this:

  INC colPntr
  colPntr.5 = 0

---------- End of Message ----------


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