The Program (6502 assembler):
  0100 9a 23    lda #23    Load the byte value 23h into the accumulator
  0102 8a 43 20 adc $2043  Add the byte of data found at memory 2043h
                           Also add one if carry flag set
  0105 84 44 20 sta $2044  Store at memory 2044h

The Fetch/Execute sequence

PC = 0102     (adc $2043 instruction  4 cycles)
  MAR<-PC     Place PC contents on address bus, signal read required.
  MBR<-M[MAR] Read opcode = 8a into data register from memory.
  PC<-PC+1    Increment PC. PC = 0103  
  IR<-MBR     Move opcode to instruction register.
              Decode opcode - indicates that next 2 bytes in memory 
              are the address of the needed data.

PC = 0103  
  MAR<-PC     Place PC contents on address bus, signal read required.
  MBR<-M[MAR] Fetch byte = 43 into data register.
  PC<-PC+1    Increment PC. PC = 0104  
  TL<-MBR     Move retrieved data into lower half of a double width 
              temporary register.

PC = 0104  
  MAR<-PC     Place PC contents on address bus, signal read required.
  MBR<-M[MAR] Fetch byte = 20 into data register.
  PC<-PC+1    Increment PC. PC = 0105  
  TH<-MBR     Move retrieved data into upper half of a double width 
              temporary register.
              Signal read.
  MAR<-TH/TL  Move combined address to memory address register.
  MBR<-M[MAR] Fetch byte at memory 2043 into data register.
  AC<-AC+MBR  Add data to Accumulator
  AC<-AC+CC[carry bit] Add carry if set.
  Change CC   Set carry flag if value overflows storage.