For clarity the PLA preprocessor allows the "word" opcode to be replaced by one or more user defined opcodes. The opcode can be used to specify the value of an OR plane field. The first step is to define an OR plane field as an opcode field. The following is an example of an opcode field.
opfld: field width=12,position=10,type=opcode
Next, each new opcode must be defined using an equate instruction, as illustrated below.
add1: equ 1
subt1: equ 2
noop: equ 3
Now, the defined opcodes may be used in place of the "word" opcode to specify a "word" statement. When the "word" opcode is used, the opcode field will be assigned its default value. When one of the new opcodes is used, the opcode field will be assigned the value of the new opcode.
Defined symbols may be used in the place of the "word" opcode, even if no opcode field has been defined. In this case, the values assigned to the opcodes are immaterial.