Save the value of AL onto the stack. This is an area of RAM starting at address BF. The stack grows towards zero. The RAM displays show the stack pointer as a blue highlight with yellow text. Push and Pop are used so that procedures and interrupts can tidy up after themselves. The procedure or interrupt can alter CPU registers but it restores them to their old values before returning.
PUSHF and POPF
PUSHF saves the CPU flags onto the stack. POPF restores the CPU flags to their original value. This enables procedures and interrupts to do useful work without unexpected side affects on the rest of the program.
IN 00
Input from port zero. This port is connected to the keyboard. The key press is stored into the AL register.
