LAX

LAX (short for "Load Accumulator and X") is the mnemonic for the illegal opcode machine language instruction which retrieves a copy from the specified RAM or I/O address, and stores it in the accumulator and the X index register. The content of the memory location is not affected by the operation.

This illegal opcode is a combination of two operations with the same addressing mode: LDA, LDX

Function: A,X = {addr}

The immediate mode LAX is an unstable opcode that ORs the A register with the "magic constant" "CONST", ANDs the result with the immediate value, and then stores the result in both A and X. CONST is chip and/or temperature dependent (common values may be $EE, $00, $FF, ...). Some dependency on the RDY line. Bit 0 and Bit 4 are “weaker” than the other bits, and may drop to 0 in the first cycle of DMA when RDY goes low. The game Wizball uses LAX in this mode in a way that assumes that CONST has a stable value of $EE when LAX is encountered, and emulators should assume this value for CONST, but real 6510 chips may have other values at different times.

Function: A,X = (A | {CONST}) & #{imm}

Addressing modes

Opcode		Addressing mode	Assembler format	Length in bytes	Number of cycles
Dec	Hex
171	AB	Immediate	LAX #nn	2	2
175	AF	Absolute	LAX nnnn	3	4
191	BF	Absolute,Y	LAX nnnn,Y	3	4*
167	A7	Zeropage	LAX nn	2	3
183	B7	Zeropage,Y	LAX nn,Y	2	4
163	A3	Indexed-indirect	LAX (nn,X)	2	6
179	B3	Indirect-indexed	LAX (nn),Y	2	5*

LAX supports 7 different addressing modes, as shown in the table at right. In the assembler formats listed, nn represents a single-byte (8-bit) figure, and nnnn is a two-byte (16-bit) address.
With some addressing forms (marked with an asterisk, *, in the "Number of cycles" column) the execution time for LAX depends on the circumstances: In cases where the indexing requires the CPU to "reach across" a page boundary from the base address, the execution time is 1 cycle longer than listed here.

CPU flags

LAX affects 2 of the CPU's status flags:

• The negative flag is set if the result is negative, i.e. has it's most significant bit set.
• The zero flag is set if the result is zero, or cleared if it is non-zero.

For the immediate mode LAX, N and Z are set according to the value of the accumulator before the instruction is executed.

Links

• NMOS 6510 Unintended Opcodes (No More Secrets)