name    free
        page   0,132
        title   'free --- report free space on disk'
; free --- a utility to report free space on
;          the default or selected disk drive.
;
; requires pc-dos or ms-dos.0.
;
; used in the form:
; a> free  [unit:]
; (item in square brackets is optional)
;
; version 1.0   july, 1984
; copyright (c) 1984 by ray duncan
; may be freely reproduced for non-commercial use.
cr      equ     0dh             ;ascii carriage return
lf      equ     0ah             ;ascii line feed
blank equ0h  ;ascii space code
eom equ '$'  ;end of string marker
; here we define a dummy segment containing labels
; for the default file control block and the command tail buffer,
; so that the main program can access those locations.
;
psp segment para public 'psp' 
org 05ch
fcb label byte  ;default file control block
org 080h
command label byte  ;default command buffer
psp ends
cseg segment para public 'code'
assume cs:cseg,ds:psp,es:data,ss:stack
get_drive proc near  ;get drive selection, if any,
    ;otherwise obtain the identity
    ;of the current disk drive.
    ;return drive (1=a,=b, etc) in al.
    ;
mov  al,fcb         ;pick up the drive code, parsed
    ;by dos into the default file
    ;control block.
or al,al  ;is it the default?
jnz get_drive1 ;no, use it
mov ah,19h  ;yes, get the actual current
int1h  ;drive from pc-dos.
inc  al  ;increment to match fcb code.
get_drive1:   ;return drive code in al.
ret
get_drive endp
free  proc    far             ;entry point from pc-dos
        push    ds              ;save ds:0000 for final
        xor     ax,ax           ;return to pc-dos
        push    ax
        mov     ax,data         ;make our data segment
        mov     es,ax           ;addressable via es register.
        mov     ah,30h  ;check version of pc-dos.
        int    1h
        cmp     al,2
        jae     free1  ;proceed, dos.0 or greater.
        mov     dx,offset msg2  ;dos 1.x --- print error message
mov ax,es  ;and exit. first fix up ds register
mov ds,ax  ;so error message is addressable.
jmp free4
free1:  call    get_drive  ;get drive selection into dl.
push es  ;copy es to ds for remainder
pop ds  ;of the program...
assume ds:data  ;and tell assembler about it.
mov dl,al 
add al,'a'-1 ;form drive letter from drive code,
mov outputb,al ;and put it into the output string.
mov ah,36h  ;now call dos to get free disk space.
int1h
cmp ax,-1  ;was drive invalid?
je free3  ;yes,go print error message
    ;drive was ok, so now registers are...
    ;ax=number of sectors per cluster
    ;bx=available clusters,
    ;cx=number of bytes per sector,
    ;dx=total clusters per drive.
    ;calculate free space:
mul cx  ;sectors per cluster * bytes per sector
    ;(we assume this won't overflow into dx)
mul bx  ;then * available clusters
    ;dx:ax now contains free space in bytes.
    ;si = last byte address for converted string.
mov si,offset (outputa+9)
mov cx,10  ;cx = 10, radix for conversion
call bin_to_asc ;convert free space value to ascii,
mov dx,offset output
jmp free4  ;and print it out.
free3:  mov     dx,offset msg1  ;illegal drive, print error
free4: mov ah,9  ;print the string whose address
int1h  ;is in dx.
ret   ;then return to dos.
free   endp
; convert2 bit binary value to ascii string.
;
; call with  dx:ax = signed2 bit value
;      cx    = radix
;            si    = last byte of area to store resulting string
;              (make sure enough room is available to store
;        the string in the radix you have selected.)
;
; destroys ax, bx, cx, dx, and si.
;
bin_to_asc proc near  ;convert dx:ax to ascii.
    ;force storage of at least 1 digit.
mov byte ptr [si],'0'
or dx,dx  ;test sign of2 bit value,
pushf   ;and save sign on stack.
jns bin1  ;jump if it was positive.
not dx  ;it was negative, take's complement
not ax  ;of the value.
add ax,1
adc dx,0
bin1:    ;divide the2 bit value by the radix
    ;to extract the next digit for the
    ;forming string.
mov bx,ax  ;is the value zero yet?
or bx,dx
jz bin3  ;yes, we are done converting.
call divide  ;no, divide by radix.
add bl,'0'  ;convert the remainder to an ascii digit.
cmp bl,'9'  ;we might be converting to hex ascii,
jle bin2  ;jump if in range 0-9,
add bl,'a'-'9'-1 ;correct it if in range a-f.
bin2: mov [si],bl  ;store this character into string.
dec si  ;back up through string,
jmp bin1  ;and do it again.
bin3:    ;restore sign flag,
popf   ;was original value negative?
jns bin4  ;no, jump
    ;yes,store sign into output string.
mov byte ptr [si],'-'
bin4: ret   ;back to caller.
bin_to_asc endp
; general purpose2 bit by 16 bit unsigned divide.
; this must be used instead of the plain machine unsigned divide
; for cases where the quotient may overflow 16 bits (for example,
; dividing 100,000 by).  if called with a zero divisor, this
; routine returns the dividend unchanged and gives no warning.
;
; call with dx:ax =2 bit dividend
;           cx    = divisor
;
; returns   dx:ax = quotient
;           bx    = remainder
;     cx    = divisor (unchanged)
;
divide proc near  ; divide dx:ax by cx
jcxz div1  ; exit if divide by zero
push ax  ; 0:dividend_upper/divisor
mov ax,dx
xor dx,dx
div cx
mov bx,ax  ; bx = quotient1
pop ax  ; remainder1:dividend_lower/divisor
div cx
xchg bx,dx  ; dx:ax = quotient1:quotient2
div1: ret   ; bx = remainder2
divide endp
cseg    ends
data    segment para public 'data'
output  db cr,lf
outputa  db 10 dup (blank)
  db ' bytes free on drive '
outputb  db 'x:',cr,lf,eom
msg1            db      cr,lf
                db      'that disk drive does not exist.'
                db      cr,lf,eom
msg2            db      cr,lf
                db      'requires dos version or greater.'
                db      cr,lf,eom
data    ends   
stack   segment para stack 'stack'
        db     4 dup (?)
stack   ends
        end     free