PIC 16F877 programmed as a digital clock

program RTC_Timer0_FreeRun_18F452

' 24 Hour Real Time Clock - Zero-Sum Timer FreeRun Method
' mikroBasic 5.02 ... 18F452 @ 8MHz Default Configuration
' by Warren Schroeder
' Timer0 in 8-bit mode; No Prescaler; 2 counts per microsecond
'
' 1 second = 2000000 clock cycles - 2 for clock restart

const OneSecond as longint = 2000000 - 2
dim Cyc_Counter as longint absolute $30   ' Timer0 cycle counter
dim T0Load as byte absolute $30           ' Timer0 reload value
dim Countdown as word absolute $31        ' # of 8-bit cycles
dim update as boolean                     ' update output
dim secs, mins, hrs as byte               ' tally variables
dim clk as string[9] absolute $20         ' output string for LCD
dim hstr as string[3] absolute $20
dim mstr as string[3] absolute $23
dim sstr as string[3] absolute $26

sub procedure interrupt
   If dec(Countdown) = 0 Then
      Cyc_Counter = OneSecond             ' reload timer
     TMR0L = TMR0L + T0Load              ' include additional TMR0 counts
      If Status.C = 0 Then
         inc(Countdown)                   ' if no carry increment counter once
      End If
      update = true                       ' flag to update clock output
   End If
   INTCON.TMR0IF = 0                      ' clear interrupt flag
end sub

sub procedure Clock24
    If inc(secs) = 60 Then                ' check each tally for rollover
       secs = 0
       If inc(mins) = 60 Then
          mins = 0
          If inc(hrs) = 24 Then hrs = 0 End If
       End If
    End If
    bytetostr(hrs, hstr)                  ' create clock output string for LCD
    If hstr[1] = 32 Then hstr[1] = 48 End If
    bytetostr(mins, mstr)
    mstr[0] = ":"
    If mstr[1] = 32 Then mstr[1] = 48 End If
    bytetostr(secs, sstr)
    sstr[0] = ":"
    If sstr[1] = 32 Then sstr[1] = 48 End If
    LCD_CMD(130)
    LCD_OUT_CP("24-HOUR CLOCK")
    LCD_CMD(195)
    LCD_OUT_CP(clk)
    update = false
end sub

sub procedure Initialize
   ADCON1 = 15
   secs = 255
   mins = 0
   hrs = 0
   update = true
   Cyc_Counter = OneSecond
   inc(CountDown)
   LCD_INIT(PORTB)
   LCD_CMD(LCD_CURSOR_OFF)
   INTCON.GIE = 1                         ' enable GIE
   INTCON.PEIE = 1                        ' enable PEIE
   T0CON = 72                             ' 8-bit; no prescaler; Timer0 off
   TMR0L = 0                              ' clear timer1
   INTCON.TMR0IE = 1                      ' interrupt enabled
   INTCON.TMR0IF = 0                      ' interrupt flag cleared
   T0CON.TMR0ON = 1                       ' start Timer0
end sub

main:
   Initialize

   While true                             ' loop forever
      If update = true Then               ' advance seconds?
         Clock24                          ' yes...then output
      End If
   Wend

end.