Shareware Grab Bag/Shareware Grab Bag.iso/001/comzap.arc/COMZAP.ASM

Assembly Source File | 1985-07-08 | 11KB | 343 lines

    page    60,132
    title    COMZAP.ASM
;...................
;Author:  Edward Nisley
;Entered from July 1985 issue of PC Tech Journal
;...................
;Program to force DOS 2.0 and 2.1 to reload COMMAND.COM
; using the file name specified by SET COMSPEC=
;Particularly useful with Vdisks and non-bootable
; fixed disks.
;
;The DOS 2.0 and 2.1 SET COMSPEC= command does not work as
; documented.  DOS will always attempt to reload COMMAND.COM
; from the root directory of the disk drive used during
; system initialization (the boot disk).
;
;This utility finds the string A:\COMMAND.COM in DOS
; and replaces it with the file name set by SET COMSPEC=
;
;How it's done:
; the BIOS memory-size interrupt determines the
; range of RAM searched for the string.
;The string A:\COMMAND.COM always starts at an
; offset address xxx9, with a binary zero byte at
; offset xxx8 just preceding it.  These observations
; are used to distinguish the true DOS file name from
; the string elsewhere (as in the program.COM file
; a VDISK or the program as loaded in RAM)
;[Or from same in the keyboard buffer under DEBUG.]
;Offset 2C in the PSP contains the segment address of
; the environment area passed to this program.  The
; COMSPEC= string is located within the envrionment.
;The COMSPEC= string is copied to the DOS string area
; and displayed on the screen as it is being copied.
;If the COMSPEC= string is too long, your PC is dead!
;The string is truncated to fit the largest space
; available, which means that it cannot be used to
; reload the command interprteer.  Power-off time!
;
;Assumptions:
;-DOS 2.0 or DOS 2.1
;-Power-on boot drive is "A"
;-You have enough RAM in your system that the
; string is not in the last segment.  The last segment
; is not checked, except in 64K systems.
;-The SET COMSPEC string is not too long.  The maximum
; length is one less than the number defined in cspecmax.
;
;Exit:  DOS function 4CH is used to pass a return code
; code 0=success!
; code 1=DOS file name string wasn't found
; code 2=COMSPEC string wasn't found
; code 3=COMSPEC string was too long, truncated
;
page
;...........................
;hocus pocus to start up the assembler
; and get the addresses correct for a .COM file
comseg    segment    'codeseg'
    assume    cs:comseg,ds:comseg,ss:comseg,es:comseg
comzap    proc
    org    100H
start:    jmp    around
;.........................
;The file name we are trying to find.  The leading
; 0FFH byte ensures that this file name isn't preceded
; by a binary zero.  This prevents incorrect matching
; when Murphy puts it at the correct offset.
    db    0FFH
bootID    db    'A'        ;the normal boot disk
CCstr    db    ':\COMMAND.COM'    ;last part of string
CCstr_l    equ    $-CCstr        ;string length
;.........................
;The COMSPEC= string header in the environment area
specstr    db    'COMSPEC='
specstr_l    equ    $-specstr    ;string length
;.........................
;Various & sundry constants
FN_hexit    dw    0009H    ;file name offset hexit
env_seg_at    equ    002CH    ;address of environment segment
prt_chr    equ    02H        ;DOS single-character output
prt_str    equ    09H        ;DOS print-string function
quit    equ    4CH        ;DOS termination function
mem_sz    equ    12H        ;BIOS memory-size interrupt
dosint    equ    21H        ;DOS function interrupt
cr    equ    0DH        ;useful characters
lf    equ    0Ah
term    equ    '$'        ;the prt_str terminator
;.....................
;A macro to simplify the DOS interface
DOScall    macro    DOS_fn
    mov    ah,DOS_fn
    int    dosint
    endm
page
;.......................
;Variables
FN_at    label    dword        ;seg:offset address of
FN_off    dw    0        ;DOS file name string
FN_seg    dw    0
cspec_at    label    dword    ;seg:offset of COMSPEC=
cspec_off    dw    0    ;string in enfironment
cspec_seg    dw    0
cspecmax    equ    39    ;longest string allowed
                ; includes trailing zero
last_seg    dw    0    ;last segment to check
;.......................
;Messages
Msg0    db    'COMZAP: a DOS fixit utility',cr,lf
    db    'Revised 4 April 85',cr,lf
    db    cr,lf
    db    'Searching for A:\COMMAND.COM...'
CrLf    db    cr,lf,term
Msg1    db    ' >> not found.',cr,lf
    db    ' >> Remember that COMZAP.COM can'
    db    cr,lf
    db    ' >> be run only once per cold boot'
    db    cr,lf,term
Msg2    db    cr,lf
    db    'The DOS command interpreter will be'
    db    'reloaded from',cr,lf
    db    '   ',term
Msg3    db    'Searching for COMSPEC= string...'
    db    cr,lf,term
Msg4    db    ' >> not found.',cr,lf
    db    ' >> Did you use SET COMSPEC=filespec'
    db    cr,lf
    db    ' >> before running COMZAP.COM?'
    db    cr,lf,term
Msg5    db    cr,lf
    db    ' >> WARNING:  COMSPEC string too long,'
    db    cr,lf
    db    ' >> DOS will not reload the interpreter'
db    cr,lf,term
page
;........................
;Start of the program code
;
;Show some identification
around:    cld            ;set increment mode
    mov    dx,offset Msg0    ;show ID message
    DOScall    prt_str
;.........................
;Get storage size and convert to segment address.
;Because the last segment may not be full, we will not
; test it for the string.
;Because we are testing in units of 64K (full segments)
; only the high-order hexit of the segment address
; is useful in comparisons.
;64K PC's (are there any left?) are special-cases.
    int    mem_sz        ;storage in 1K units
    mov    cl,6        ;shift MSB to bit 15
    shl    ax,cl
    and    ax,0F000H    ;isolate 64K seg hexit
    jz    oneseg        ;handle 64K machines
    sub    ax,01000H    ;set last segment
oneseg:    mov    last_seg,ax
;........................
;Search for the starting "A" in "A:\COMMAND.COM"
;The inner loop uses CMP to check the 4096 offset
; addresses ending in the hexit 9 in each segment.
;The outer loop verifies inner-loop hits and ticks
; the segment addresses at the end of segments
    mov    al,bootID    ;the test byte
    mov    bx,00000H    ;first segment address
    mov    es,bx        ;into segment register
    assume    es:nothing    ;tell the assembler
CClp0:    mov    si,FN_hexit    ;get the offset address
    sub    si,0010h    ;fix for pre-increment
    mov    cx,1000H    ;64K/16 = # of 9's
CClp1:    add    si,0010H    ;tick offset pointer
    cmp    al,es:[si]    ;test the byte
    loopne    CClp1        ;loop if not equal
;.....................
;Inner loop ended:
; if "not equal", the entire segment has been examined
; without finding a qualifying "A", so go to next seg
    jne    nextseg
;......................
;We've found an "A" at an offset xxx9
; if it's preceded by a binary zero, look closer...
    cmp    byte ptr es:[si-1],00H
    je    testmore    ;hmm... looking good!
page
;.......................
;The "A" doesn't pass muster, so continue with the
; next byte in the segment.
;If CX is zero, all bytes have been tested,
; so we must step to the next segment
    cmp    cx,0
    jne    CClp1
    jmp    nextseg
;......................
;An "A" and a binary zero have been located!!!
;Time to compare the strings.
;Save the current state, just in case it's a miss
testmore:
    push    si        ;save current offest
    push    es        ;  ... segment
    push    cx        ;  ... count
    push    ax        ;  ... test byte
;.................
;Set the segment address to the paragraph just before
; the string to avoid overrunning the segment boundary
;This is complicated a bit by the restricted nature of
; the 8088's "general" registers
    mov    bx,si        ;get current offset
    mov    cl,4        ;convert to paragraphs
    shr    bx,cl
    mov    cx,es        ;add the segment
    add    bx,cx
    mov    es,bx        ;set in in seg reg
    mov    FN_seg,bx    ;save segment
;.....................
;Set up the offset addresses to the known string and
; the candidate we just found
;The candidate offset is always FN_hexit by definition
    mov    di,FN_hexit    ;known candidate offset
    mov    FN_off,di    ;save offset
    inc    di        ;step over "A"
    mov    si,offset CCstr    ;known string
    mov    cx,CCstr_l    ;  ... its length
;....................
;After all that, this is the string com0parison.
;Registers are restored before the branch simply to
; avoid a double branch.
;The CMPS operands are coded for human readability.
;Failure returns us to the inner test loop above.
;Success passes us to the COMSPEC= string eaarch.
    repe    cmps    byte ptr ds:[si],es:[di]
    pop    ax        ;restore saved regs
    pop    cx
    pop    es
    pop    si
    jne    CClp1        ;no good, continue
    jmp    gotfile        ;swell, break out
page
;.....................
;outer loop ending...
;Didn't find the string in this segment
;Trick the segment until we've completed the search.
;Remember that the RAM in the last segment will not
; be examined because the segment may not be filled
; with RAM...
;A tige more code would make the search complete.
nextseg:
    mov    bx,es        ;tick the segment pointer
    add    bx,1000H
    mov    es,bx
    cmp    bx,last_seg    ;done?
    jbe    CClp0        ;nope, back to loops
    mov    dx,offset Msg1    ;present error message
    DOScall    prt_str
    mov    al,1        ;return code
    DOScall    quit
page
;......................
;Tah-DAH!  Found the DOS file specifier in RAM.
;Now we have to look for the COMSPEC string.
;Each string in the environment is terminated with a
; binary zero.  The end of the environment area is
; marked by two binary zeros.
;Note how the "general" pointer registers aren't...
gotfile:
    mov    dx,offset Msg3    ;say where we are
    DOScall    prt_str
    mov    si,env_seg_at    ;points to env seg ptr
    mov    es,[si]        ;pick up segment addr
    mov    cspec_seg,es    ;save seg addr
findspec:
    mov    di,cspec_off    ;get offs addr
    cmp    byte ptr es:[di],00h    ;at end?
    je    nospec        ;sigh, give up
    mov    si,offset specstr    ;test string
    mov    cx,specstr_l
    repe    cmps    byte ptr ds:[si],es:[di]
    je    gotspec        ;kapow!
    mov    si,di        ;use right "general" reg
junkit:    lods    byte ptr es:[si]    ;scan for end
    cmp    al,00H
    jne    junkit
    mov    cspec_off,si    ;set up pointer
    jmp    findspec    ;and try again
;..........................
;Didn't find a COMSPEC= string in the environment.
;Return to DOS with an error code.
nospec:
    mov    dx,offset Msg4    ;show a sign
    DOScall    prt_str
    mov    al,2        ;set return code
    DOScall    quit
page
;......................
;Found the COMSPEC= string in the enfironment area
;ES:DI now points to the start of the file spec string
;Need to get source in DS:SI and target in ES:DI
;Transfer the COMSPEC= string to the DOS file string
; and display it as it is transferred.
;The trailing binary zero is transferred as well, to
; overwrite any previous file name you may have
; manually inserted into the DOS file string while
; debugging this program...
;The string will be truncated to prevent stamping on
; other DOS data following the file name area.
gotspec:
    mov    cspec_off,di    ;save offset
    mov    dx,offset Msg2    ;say what's happening
    DOScall    prt_str
    mov    cx,cspecmax    ;max length allowed
    les    di,FN_at    ;DOS string address
    lds    si,cspec_at    ;COMSPEC string addr
xfer:    lods    byte ptr ds:[si]    ;COMSPEC char
    stos    byte ptr es:[di]    ;to DOS char
    cmp    al,00H        ;hit the end?
    je    alldone        ;yup, quit
    mov    dl,al        ;nope, show it
    DOScall    prt_chr
    loop    xfer        ;repeat for count
                ;if fall through, error!
    mov    ax,cs        ;restore DS
    mov    ds,ax
    mov    dx,offset Msg5    ;tell them about truncation
    DOScall    prt_str
    mov    al,3        ;set return code
    DOScall    quit
;..................
;Done with transfer and display, return to DOS
alldone:
    mov    ax,cs        ;restore DS
    mov    ds,ax
    mov    dx,offset crlf    ;insert some space
    DOScall    prt_str
    mov    al,0        ;good return code
    DOScall    quit        ;and exit
page
;....................
;hocus pocus to turn off the assembler
; and set up .COM starting address
comzap    endp
comseg    ends
end    start