; 99 Bottles of Beer program in Zilgo Z80 assembly language.
;  Assembles for ZX Spectrum/Timex - change Spectrum: lines 
;  if porting.  Help from Marko!  Compiled and tested with SPIN 
; 
; Adapted from the Alan deLespinasse's Intel 8086 version 
; 
; Author: Damien Guard ; damien@envytech.co.uk ; www.damieng.com

 org 32768

start:
 ld      a, 2                  ; Spectrum: channel 2 = "S" for screen
 call    $1601                 ; Spectrum: Select print channel using ROM

 ld c,99                       ; Number of bottles to start with


loopstart:
 call printc                   ; Print the number of bottles
 ld hl,line1                   ; Print the rest of the first line
 call printline

 call printc                   ; Print the number of bottles
 ld hl,line2_3                 ; Print rest of the 2nd and 3rd lines
 call printline

 dec c                         ; Take one bottle away
 call printc                   ; Print the number of bottles
 ld hl,line4                   ; Print the rest of the fourth line
 call printline

 ld a,c
 cp 0                          ; Out of beer bottles?
 jp nz,loopstart               ; If not, loop round again

 ret                           ; Return to BASIC


printc:                        ; Routine to print C register as ASCII decimal
 ld a,c
 call dtoa2d                   ; Split A register into D and E

 ld a,d                        ; Print first digit in D
 cp '0'                        ; Don't bother printing leading 0
 jr z,printc2
 rst 16                        ; Spectrum: Print the character in 'A'

printc2:
 ld a,e                        ; Print second digit in E
 rst 16                        ; Spectrum: Print the character in 'A'
 ret


printline:                     ; Routine to print out a line
 ld a,(hl)                     ; Get character to print
 cp '$'                        ; See if it '$' terminator
 jp z,printend                 ; We're done if it is
 rst 16                        ; Spectrum: Print the character in 'A'
 inc hl                        ; Move onto the next character
 jp printline                  ; Loop round

printend:
 ret


dtoa2d:                        ; Decimal to ASCII (2 digits only), in: A, out: DE
 ld d,'0'                      ; Starting from ASCII '0' 
 dec d                         ; Because we are inc'ing in the loop
 ld e,10                       ; Want base 10 please
 and a                         ; Clear carry flag

dtoa2dloop:
 inc d                         ; Increase the number of tens
 sub e                         ; Take away one unit of ten from A
 jr nc,dtoa2dloop              ; If A still hasn't gone negative, do another
 add a,e                       ; Decreased it too much, put it back
 add a,'0'                     ; Convert to ASCII
 ld e,a                        ; Stick remainder in E
 ret

; Data
line1:    defb ' bottles of beer on the wall,',13,'$'
line2_3:  defb ' bottles of beer,',13,'Take one down, pass it around,',13,'$'
line4:    defb ' bottles of beer on the wall.',13,13,'$'