6502 bootstrap - a CPU detour

July/august 1984 a CPU detour During its initialization procedure, the 6502 processor starts by getting the start vector which is located at ad dresses $FFFC and $FFFD in ROM. This is a fixed instruction that cannot be changed, and it points to a memory zone in PROM, which, in most computers, is very difficult for the user to access. The circuit described here makes it easy to reroute the 6502 to a start address chosen by the user: $XFFC/$XFFD where X is any hexadecimal value. At this address the CPU will find the appropiate vector pointing to the start routine written by the user (in EPROM) instead of the standard routine written by the manufacturer. The only hardware change required to achieve this is to connect the cir cuit shown between the 6502 and its bus. Now every time the CPU emits an address between $FFF8 and $FFFF (the address decoding is a little less precise than is necessary for only re-routing the processor when it outputs addresses $FFFC and $FFFD), the bus receive...