Dubious features of 6052

The original 6502 and its NMOS derivatives are noted for having a variety of undocumented instructions, which vary from one chip manufacturer to the next. The 6502's instruction decoding is implemented in a hardwired logic array (similar to a programmable logic array) which is only defined for 151 of the 256 available opcodes. The remaining 105 trigger strange and hard-to-predict actions (e.g., immediately crashing the processor, performing several valid instructions at once, or simply doing nothing at all). Eastern House Software developed the "Trap65", a device that plugged between the processor and its socket to convert (trap) unimplemented opcodes into BRK (software interrupt) instructions. Some programmers utilized this feature to extend the 6502's instruction set by providing functionality for the unimplemented opcodes with specially written software intercepted at the BRK instruction's 0xFFFE vector. All of the undefined opcodes have been replaced by NOP instructions in the 65C02 CMOS version (although with varying byte sizes and execution times).

The 6502's memory indirect jump instruction, JMP (

), is partially broken. If

was hex xxFF (i.e. any word ending in FF), the processor would not jump to the address stored in xxFF and xxFF+1, but rather the one in xxFF and xx00. This defect continued through the entire NMOS line, but was fixed in the CMOS derivatives.

The N (result negative), V (sign bit overflow) and Z (result zero) status flags are not valid when performing arithmetic operations while the processor is in BCD mode, as these flags reflect the binary, not BCD, result. This limitation was removed in the CMOS derivatives. Therefore, this feature may be used to cleanly distinguish CMOS from NMOS CPU versions without using any illegal opcodes.

If the processor happens to be in BCD mode when a hardware interrupt occurs it will not revert to binary mode. This quirk could result in hard-to-solve bugs in the interrupt service routine if it failed to clear BCD mode before performing any arithmetic operations. For example, the Commodore 64's kernel did not correctly handle this processor characteristic, requiring that IRQs be disabled or revectored during BCD math operations. This issue was addressed in the CMOS derivatives as well.

The SO pin (Set Overflow) was intended for use in high-speed device drivers. Asserting it would immediately set the processor's Overflow (V) status register bit. Successful use of this feature could eliminate a load instruction from a high-speed device driver, reducing the number of instructions in a data transfer loop by 25%, but obviously great care was required in the system design in order not to corrupt general computation.

The 6502 instruction set includes BRK (opcode $00), which is technically a software interrupt (similar in spirit to the SWI mnemonic of the 6800). BRK is most often used to interrupt program execution and start a machine code monitor for testing and debugging during software development. It could also be used to route program execution using a simple jump table (analogous to the manner in which the 8088 and derivatives handle software interrupts by number). Unfortunately, if a hardware interrupt occurs at the same time the processor is fetching a BRK instruction, the NMOS version of the processor will fail to execute BRK and instead proceed as if only a hardware interrupt had occurred. This fault was corrected in the CMOS implementation of the processor.

The JSR (call subroutine) instruction pushes the address of the last byte of the call instruction on to the stack (the program counter would have been increased after execution has been done) . The RTS (return) instruction pulls the return address off the stack and increments it before placing it into the program counter, resulting in automatic compensation for this design quirk. This characteristic would go unnoticed unless you pulled the return address to pick up parameters in the code stream (a common 6502 programming idiom). It remains a characteristic of 6502 derivatives to this day.