Free of Assembly Source series

This is the past DOS era of the compilation of source code, although has passed, but for the study of the assembly is still helpful, assembly language is just a basic programmer language, most people can grasp, not necessarily in-depth research.

Name free
Page 60,132
Title ' Free---The free spaces on disk '
; Free---a utility to the
; The default or selected disk drive.
;
; Requires Pc-dos or MS-DOS 2.0.
;
; Used in the form:
; a> free [unit:]
; (item in square brackets is optional)
;
; Version 1.0 July 4, 1984
; Copyright (c) 1984 by Ray Duncan
; May is freely reproduced for non-commercial use.
CR Equ 0DH; ASCII Carriage return
LF equ 0ah; ASCII line Feed
Blank Equ 20h; 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 this main program can access those locations.
;
PSP segment para Public ' PSP '
ORG 05ch
FCB Label byte;d efault file control block
ORG 080h
Command label byte;d efault 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
; The current disk drive.
; Return Drive (1=a, 2=b, etc.) in AL.
;
MOV AL,FCB; Pick up the drive code, parsed
; by DOS into the default file
;
Or al,al is it the default?
JNZ get_drive1; no, use it
MOV ah,19h; Yes, get the actual current
int 21h;d rive 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 21h
CMP al,2
Jae Free1;p roceed, DOS 2.0 or greater.
mov dx,offset msg2;D OS 1.x---print error message
mov ax,es and exit. A/fix up DS register
MOV ds,ax so error message is addressable.
JMP free4
Free1:call get_drive to drive selection into DL.
Push es; copy es to DS for remainder
Pop ds;
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 spaces.
int 21h
CMP ax,-1 was drive invalid?
Je free3; yes,go print error message
;d Rive is OK, so now registers are ...
; Ax=number of sectors per cluster
; Bx=available clusters,
; Cx=number of bytes per sector,
;D x=total clusters per drive.
; Calculate free Spaces:
Mul CX; Sectors per cluster * bytes per sector
;(we assume this won ' t overflow into DX)
Mul BX; then * available clusters
;D X:ax now contains free spaces in bytes.
; SI = last byte to converted string.
mov Si,offset (outputa+9)
MOV cx,10; CX = ten, radix for conversion
Call Bin_to_asc convert free spaces 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;p Rint the string whose address
int 21h; is in DX.
RET; then return to DOS.
Free ENDP
; Convert bit binary value to ASCII string.
;
; Call with Dx:ax = signed bit value
; CX = Radix
; SI = Last byte of area to store resulting string
; (Make sure enough room are available to store
; The string in the radix your 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 of bit value,
PUSHF and save sign on stack.
Jns Bin1 Jump if it was positive.
Not DX; It is negative, take 2 ' s complement
not ax; the value.
Add ax,1
ADC dx,0
Bin1:;d ivide The 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 to caller.
BIN_TO_ASC ENDP
; General purpose bit by bit unsigned divide.
; This must is used instead of the plain machine unsigned divide
; For cases where the quotient may overflow bits (for example,
;　Dividing 100,000 by 2). If called with a zero divisor, this
; Routine returns the dividend unchanged and gives no warning.
;
; Call with Dx:ax = 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 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 2 or greater. '
DB Cr,lf,eom
Data ends
Stack segment para stack ' stack '
DB DUP (?)
Stack ends
End Free