topic Re: program counter & reset vector in 8-bit Microcontrollers

program counter & reset vector

zen — Sat, 25 Aug 2007 11:47:05 GMT

Hi,

I am a HC08 beginner. I wonder how the contents of Program Counter and RESET Vector ($FFFE,$FFFF) are set at the beging of a program. Below is where the question stems from.

I created a project with CW. In the automatic created file " INCLUDE 'derivative.inc" has this code:

ROMStart: equ $00008000

INT_RESET: equ $0000FFFE

in the automatic created main.asm has this code:

; Include derivative-specific definitions

INCLUDE 'derivative.inc'

      ; export symbols
            XDEF _Startup
            ABSENTRY _Startup

; variable/data section
ORG RAMStart ; Insert your data definition here

ExampleVar: DS.B 1

TEMP1: EQU $0040

; code section

ORG ROMStart

_Startup:
             LDHX   #RAMEnd+1        ; initialize the stack pointer
            TXS
            CLI                     ; enable interrupts

mainLoop:

            ; Insert your code here
            NOP

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

When simulating this program in CW, I found that both the contents of PC (program counter) and $FFFE,$FFFF are 8000. I wonder how each content of the two is set?

Thanks!

zen

Re: program counter & reset vector

rocco — Sat, 25 Aug 2007 15:31:23 GMT

Hi Zen,

The RESET vector, $FFFe:#FFFF, is in flash memory, and is programmed to point to the starting address of your code when you program your flash memory.

Then, as the chip comes out of RESET, the contents of $FFFE:$FFFF are copied into the program counter, to cause you program to start execution at the starting address of your code.

Hope that helps.

Re: program counter & reset vector

CompilerGuru — Thu, 29 Oct 2020 08:44:26 GMT

if the question is where the 0xfffe bytes come from, then look further down the main.asm.

For absolute assembly projects with no device init support,
is the reset vector set further down in main.asm

Code: end of main.asm

            ORG   $FFFA            DC.W  spurious          ;            DC.W  spurious          ; SWI            DC.W  _Startup          ; Reset

Daniel

Re: program counter & reset vector

zen — Sat, 25 Aug 2007 17:47:33 GMT

Thank you, rocco and Daniel.

I realize now that the initial content at $FFFE,$FFFF are set at the time of programing the flash memory by some kind of progrmmer, and are determined by

"DC.W Startup ; Reset"

after programming the chip and power off the chip, the content of Reset vector still keep its value as being programmed.

Now, my remain question is about the content of PC. PC is in CPU, not in flash memory, so it is impossible to set its value as progrmming the chip. If each time powering on the chip generates a Reset condition, then it is understandable that the content at $FFFE,$FFFF will be loaded to PC. But it is hard for me to understand that the assumption is reasonable.

Thanks

zen

Re: program counter & reset vector

zen — Sat, 25 Aug 2007 21:46:59 GMT

From a reference book, I find that, power-on does create a reset, called Power-on Reset, POR, the result is that PC is loaded with the content of Reset vector. It should be where your porgram begins.