;
; Procedures written in assembly but called by C
; 16-bit (real mode) version!
;
; C uses the stack to pass parameters, pushing them in reverse order
;
; We use a STANDARD STACK FRAME to access the stack, by PUSHing BP,and
; setting it to SP:
;
; STACK
;--------
; arg3    (SP+10)
; arg2    (SP+8)
; arg1    (SP+6)
;  CS     (SP+4)
;  IP     (BP+2)
;  BP     (BP=SP)
;  ...

;
; The return value is put in
;
; AL    - if it's a "char" (byte)
; AX    - if it's an "int" (word)
; DX:AX - if it's a "long" (dword)

; Any procedure called by C:
;
;    1) cannot modify ES,DS,SI,DI,BP
;    2) must balance the stack
;    3) must begin with an underscore "_" (name mangling)
;    4) must initialize DS to use its own ASM variables


;
; More examples:
;

_MYSUB
   push bp
   mov bp,sp

   push ds         ; Save registers

   mov ax,cs       ; Load DS
   mov dx,ax

   mov ax,[bp+6]   ; Access function parameters
   sub ax,[bp+8]   ; return value in AX
   
   pop ds          ; Restore registers

   pop bp
   retf


;
; int printchar(int num,char c);
;

_printchar
   push bp
   mov bp,sp
   ..
   mov cx, [bp+6]
   mov dl, [bp+8]    ; character occupies lower address
top:
   call dspout
   loop  top
   ..
   pop bp
   retf


;
; int mydiv(long dividend,int divisor);
;
; [BP+8]:[BP+6] - dividend
; [BP+10]       - divisor
;
_mydiv
   push bp
   mov bp,sp
   ..
   mov dx,[bp+8]        ; example of a dowubleword argument spread
   mov ax,[bp+6]        ; across two stack locations

   div word [bp+10]     ; stack locations can be used as operands
   ..
   pop bp
   retf


;
; char mystr="Hello, how are you today!?$"
; printstr(mystr);
;


;
; void printstr(char *str);
;
; [BP+6] - offset of string to print
;
_printstr
   push bp
   mov bp,sp         ; set standard stack frame
   ..
   mov si,[bp+6]     ; Offset of message
   ..
PrintLoop:
   mov dl,[si]       ; Read character from [DS:offset]
   cmp dl,'$'
   je PrintLoopDone  
   call dspout
   inc SI
   jmp PrintLoop
   ..
PringLoopDone:
   pop bp
   retf



;
; PrintTheAnswer("fifty-seven");
;

;
; void PrintTheAnswer(char *str);
;
;   Like above, but prepends output with 'The Answer is: '
;
; [BP+6] - offset of string to print
;

OutMessage db 'The Answer is: ',$

_PrintTheAnswer
  push bp
  mov bp,sp

  push ds                     ; Must preserve default segment in C call

  mov ax,cs
  mov ds,ax                   ; Load default segment for OutMessage

  mov ax, OutMessage  
  push ax                     ; Push offset of OutMessage 
  Call _printstr              ; Print 'The Answer is: '
  pop ax                      ; cleanup stack

  pop ds                      ; restore DS

  mov ax, [bp+6]
  push ax
  Call _printstr              ;  Print acutal message
  pop ax

  pop bp
  retf