setmode: sei ;no IRQs
sec ;default the...
xce ;MPU
clc ;return to...
xce ;native mode
cld ;binary arithmetic
...etc...
Your method of initializing the 65C816 is always assuming the MPU is in emulation mode. That would be the case if powering on or if a hard reset occurs. However, if your ...

;PURPOSE - This is the code which will written to ROM at $0000 (but will be referenced as $8000)
;
;
;The first difference we'll see is the inclusion of other source files (note they do not have .s extensions, we do not want
;them assembled as their own objects).
; ca65 documentation: https://cc65 ...

Had my "Hello World", code running successfully the other day. Added some additional functionality to initialize my ACIA and output data to the serial port. However now my assembled hex dump doesn't seem to make sense. I normally start things off by setting the stack pointer LDX #FF and STX opcode ...