SBX

SBX (short for "SuBtract and store in X") is an illegal opcode that ANDs the contents of the A and X registers (leaving the contents of A intact), subtracts an immediate value, and then stores the result into X.

This illegal opcode is a combination of an immediate and an implied command: CMP, DEX

As the subtraction is performed in the CPU based on the CMP function, this subtract operation is not affected by the state of the Carry Flag, doesn't change the state of the Overflow Flag, and doesn't respect the decimal mode.

Function: X = A & X - #{imm}

Addressing mode

Opcode		Addressing mode	Assembler format	Length in bytes	Number of cycles
Dec	Hex
203	CB	Immediate	SBX #nn	2	2

SBX only supports the Immediate addressing mode, as shown in the table on the right.

CPU flags

SBX affects 3 of the CPU's status flags analog to CMP according to the value ending up in register X:

• The negative flag is set if the result is negative, i.e. has its most significant bit set.
• The zero flag is set if the result is zero, or cleared if it is non-zero.
• The carry flag is set or cleared depending on the result.

Examples

Decrement X by more than 1

SBX can be used in a simple trick to decrement X by more than 1:

TXA      ; A = X, so A & X = X
SBX #$xx ; where xx is the value to decrease by

Negate a 16 bit number

LAX #$00 ; A=0 and X=0
SBX #lo  ; sets carry automatically for upcoming SBC
SBC #hi  ; negated value is in A/X

This routine is especially useful if self-modifying code is used to set the values of #lo and #hi

Links

• NMOS 6510 Unintended Opcodes (No More Secrets)