ALR

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ALR (short for "AND then Logical shift Right") is a mnemonic for an illegal opcode machine language instruction.

This illegal opcode is a combination of two operations using immediate addressing mode: AND, followed by LSR

Function: A = (A & #{imm}) / 2

Addressing mode[edit]

Opcode Addressing
mode
Assembler
format
Length
in bytes
Number of
cycles
Dec Hex
75 4B Immediate ALR #nn 2 2

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

CPU flags[edit]

ALR affects 3 of the CPU's status flags according to the state after the shift:

  • The negative flag is always cleared.
  • The zero flag is set if the result is zero, or cleared if it is non-zero.
  • The carry flag has the value of accumulator's bit 0 if bit 0 of the mask is 1 or cleared otherwise.

Examples[edit]

Shift Right and Clear Carry

ALR #$FE

is equivalent to:

LSR
CLC

but takes 2 cycles instead of 4.

Links[edit]