9.1.4.1 Computed GOTO

A computed GOTO is accomplished by adding an offset to the Program Counter. An example is shown in the following code example.

A Look-up Table can be formed with an ADDWF PCL instruction and a group of RETLW nn instructions. The W register is loaded with an offset into the table before executing a call to that table. The first instruction of the called routine is the ADDWF PCL instruction. The next instruction executed will be one of the RETLW nn instructions that returns the value ‘nn’ to the calling function.

The offset value (in WREG) specifies the number of bytes that the Program Counter will advance and must be multiples of two (LSb = 0).

In this method, only one data byte may be stored in each instruction location and room on the return address stack is required.

     RLNCF   OFFSET, W   ; W must be an even number, Max OFFSET = 127
     CALL    TABLE

     ORG     nn00h    ; 00 in LSByte ensures no addition overflow
TABLE:
     ADDWF   PCL      ; Add OFFSET to program counter
     RETLW   A        ; Value @ OFFSET=0
     RETLW   B        ; Value @ OFFSET=1
     RETLW   C        ; Value @ OFFSET=2
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