VinCBR900 wrote:
Hi everyone,
If you were part of the 6502 design team, what would you have pushed for? Given the die was initially too large, any changes need to be part of a tradeoff.
Here is a suggestion to get the ball rolling:
1 bit Input/output ports like Signetics 2650, COSMAC 1802, INS8060, i8085
- Remove decimal mode to free up some space
- Change SO pin to an output FLAG pin and connect to bit 3 of Status Reg, previously Decimal Mode
- Rename the CLD/SED Clear/Set decimal flag instructions to Set/ClearFlagPin
- Connect IRQ pin to bit 5 of Status Reg - Replaces functionality of SO pin for 1 bit input
- However, Needs an instruction to either test (and set another SR bit) or branch
There is a related thread on
what could we get rid of, but this thread is what would you change based on tradeoff.
Interesting list. I wonder about the feasibility of dropping BCD support, but I'm not very familiar with the mid-70's embedded market they were primarily targeting, so not sure if it would be deal breaker.
If I knew then what we know now about dropping memory prices, I'd use some of the NC pins as an indicator of type of memory access being performed, similar to VPA/VPD on the '816, in order to allow for potential separate address spaces. Maybe a vector pull pin?
Similarly, if I knew that compiled languages were the wave of the future, I'd try to find a way to squeeze in a larger stack pointer and stack relative addressing modes, if at all possible.