Multiply Without MUL Instruction

Multiply two 8-bit numbers on a core with no hardware multiplier using shift-and-add, taking at most 8 iterations.

Constraints

• Operands in r16 and r17, 16-bit result in r18:r19.
• Use only shifts, adds and branches.
• Must terminate early when the multiplier reaches zero.
• Handle a zero operand correctly.
• AVR assembly syntax.
• Do not use the MUL instruction even if the core has one.