SUBX - M68K instruction reference

subxSubtract extended 
Subtracts the source and the extend flag from the destination.
Sizes .b .w .l, .w by default
XNZVC ●●●●●
● set from the result
Reads X
Forms  subx ,   
 subx ,   
Operands Dn A data register, d0 to d7. More
-(An) The simulator first decreases the address register by the operation’s size in bytes, then accesses memory at the new address, for example -(sp). More
Use matching data registers or matching predecrement operands for multi-precision arithmetic.