Type checking for REGISTER cause the assembler to perform type checking of register fields of machine instruction operands.

As described previously in Extensions to the DC, DS, and EQU assembler instructions, you can use the EQU assembler instruction to specify assembler types to be assigned to a symbol. The assembler types apply to registers. The REGISTER suboption (the default) controls whether the assembler uses these assembler types. To disable the checking, use the NOREGISTER suboption.

When it is resolving register fields of machine instructions, the assembler uses the machine instruction format, expected operand format, and expected operand length, to check for acceptable types on any symbols specified as register operands or register parts of operands.

The assembler maintains flags which track when one or more instances of a specific register assembler type has been encountered on an EQU instruction in the source code. The assembler uses these flags to decide on the depth of register type checking for that piece of source code.

The description and examples in the remainder of this section assume that checking is active, and describe assembler behavior when machine instruction register fields of specific types are being evaluated.