Hi Guys,

i have a question about the 6502 processor's stack pointer.

1. how big is the stack area of memory and what does it store and in what format?
2. what does the stack pointer store, a memory location(to what) or a position number within the stack area of memory(e.g. location 2)

Cheers

Tony

Tony,

The stack pointer in a 65(C)02 is implemented as an 8-bit register that provides an index into the area of memory between 0x100 and 0x1ff. This is part of the main memory map (unlike some microcontrollers that have a seperate memory map for their stacks?). It is byte aligned and contains a mixture of both 'system' and 'user' information. The system information is typically return addresses held, if memory serves, as low-byte then high-byte. Obviously user information can be in any format.

I hope this helps.

Regards,

Simon J

Tony,

The stack pointer in a 65(C)02 is implemented as an 8-bit register that provides an index into the area of memory between 0x100 and 0x1ff. This is part of the main memory map (unlike some microcontrollers that have a seperate memory map for their stacks?). It is byte aligned and contains a mixture of both 'system' and 'user' information. The system information is typically return addresses held, if memory serves, as low-byte then high-byte. Obviously user information can be in any format.

I hope this helps.

Regards,

Simon J

If I remember correctly, when a JSR instruction is executed, the current program counter is pushed on the stack + or - 2 (something strange like that). Why isn't the return adddress (JSR + 3) put onto the stack for eventual RTS?

I've always wanted to know this.

Jim Lewandowski

Hi Jim,

You said ...

If I remember correctly, when a JSR instruction is executed, the current program counter is pushed on the stack + or - 2 (something strange like that). Why isn't the return adddress (JSR + 3) put onto the stack for eventual RTS?

....

The return address-1 is pushed (or JSR +2 whichever you prefer to call it). Why? Well after the return address is pulled from the stack the program counter is incremented by one before the next instruction is fetched.

You know it all makes sense.

Lee.