CD ROM Paradise Collection 4/CD ROM Paradise Collection 4 1995 Nov.iso/music/c2snd201.zip/PPWAV.ASM

Assembly Source File | 1994-06-11 | 29KB | 1,497 lines

; PPWAV.ASM
;
; PreProcess Wav:  this program performs one or more of the following
; conversions on a .wav file:  mix to mono, convert to 8-bit, and halve
; sampling rate.  This program is provided for users of PLAYWAV who have
; been confonted with the "Output underflow" message.  Its purpose is to
; reduce the size of a .wav file so that it can be played on limited
; hardware (such as a floppy-only system, or to a lesser extent the SL).
;     Note that mixing to mono and converting to 8-bit with PPWAV do not
; affect the sound quality when the .wav is to be played on a Tandy with
; PLAYWAV, since those conversions must be done when the .wav is played
; anyway.
;     Some unusual .wav types that PLAYWAV can't play can also be con-
; verted with this program (more than 16 bits per channel, more than 2
; channels, sampling rate higher than 65535 Hz).
;     The input file is copied rather then overwritten.
;     This program is in .exe format due to memory requirements.
;     Syntax is as follows:
;
;     PPWAV <input file> <output file>
;
; There are no options.  The user will be prompted for input from the key-
; board.
;
; Order of segments:
;

SCODE    SEGMENT
SCODE    ENDS
SDATA    SEGMENT
SDATA    ENDS
    ;
    ; Stack segment.
    ;
STACK    SEGMENT    STACK
    DW    512 DUP (?)
STACK    ENDS
    ;
    ; Buffer for output samples (16K).
    ;
OUTBUF    SEGMENT
    DB    16384 DUP (?)
OUTBUF    ENDS
    ;
    ; Buffer for input samples (32K).
    ;
INBUF    SEGMENT
    DB    32768 DUP (?)
INBUF    ENDS

SDATA        SEGMENT
;
; Data.
;
; Template for .wav header.  This header will be filled in to match the
; output file and written out.  The strings below are also used to verify
; the input .wav.
;
WAVHEADER    EQU    $
RIFFSTR        DB    "RIFF"        ; RIFF signature = "RIFF"
WAVLEN        DD    0        ; (length of .wav file) - 8
WAVESTR        DB    "WAVE"        ; WAVE signature = "WAVE"
FMTSTR        DB    "fmt "        ; format chunk header = "fmt "
        DD    16        ; length of format chunk = 16
        DW    1        ; format type = 1 (Microsoft PCM)
NCHANNELS    DW    0        ; number of channels
SAMPRATE    DD    0        ; samples per second
BYTESPERSEC    DD    0        ; bytes per second
SAMPSIZE    DW    0        ; bytes per (multichannel) sample
SAMPWIDTH    DW    0        ; bits per channel
DATASTR        DB    "data"        ; data block header = "data"
DATALEN        DD    0        ; number of bytes in data chunk
        ;
        ; Additional output parameters.
        ;
CHANSIZE    DW    0        ; bytes per channel
ISSIGNED    DB    0        ; 1 = output samples are signed
NSAMPLES    DD    0        ; number of samples in output file
        ;
        ; Flag, 1 = format chunk processed.
        ;
FMTDONE        DB    0
        ;
        ; Input file format parameters.
        ;
INISSIGNED    DB    0        ; 1 = input samples are signed
INNCHANNELS    DW    0        ; number of channels
INSAMPRATE    DD    0        ; sampling rate
INSAMPWIDTH    DW    0        ; bits per channel
INCHANSIZE    DW    0        ; bytes per channel
INSAMPSIZE    DW    0        ; bytes per (multichannel) sample
INNSAMPLES    DD    0        ; number of samples in input file
        ;
        ; Small buffer for reading header fields and for processing
        ; samples.
        ;
SMALLBUF    DB    16 DUP (0)
        ;
        ; Small buffers for mixing channels.
        ;
MIXBUF        DB    9 DUP (0)
CHANBUF        DB    9 DUP (0)
        ;
        ; Number of input samples needed to make 1024 output samples.
        ;
SAMPTOREAD    DW    1024        ; change to 2048 if HALVEFLAG on
        ;
        ; Number of bytes to read to get 1024 output samples.
        ;
BYTESTOREAD    DW    0
        ;
        ; Number of bytes from the input buffer that have been
        ; processed.
        ;
BYTESTAKEN    DW    0
        ;
        ; Number of bytes placed in the output buffer.
        ;
BYTESPLACED    DW    0
        ;
        ; Number of input bytes on each channel to skip over.
        ;
SKIPLENGTH    DW    0
        ;
        ; Error messages.
        ;
USAGEMSG    DB    "Usage:  PPWAV <input file> <output file>",0Dh,0Ah
        DB    "- see docs for details.",0Dh,0Ah,"$"
INOPENMSG    DB    "Error opening input file.",0Dh,0Ah,"$"
OUTOPENMSG    DB    "Unable to create output file.",0Dh,0Ah,"$"
READERRMSG    DB    "Error reading input .wav file.",0Dh,0Ah,"$"
WRITEERRMSG    DB    "Error writing output .wav file.",0Dh,0Ah,"$"
BADWAVMSG    DB    "Input .wav file invalid or unsupported type."
        DB    0Dh,0Ah,"$"
        ;
        ; File handles.
        ;
INHANDLE    DW    0        ; input file handle
OUTHANDLE    DW    0        ; output file handle
        ;
        ; Information about the input file to be displayed to the user.
        ;
MSGA        DB    "Input file:  $"
INFILENAME    DB    128 DUP (0)        ; input file name
MSGB        DB    0Dh,0Ah,9,"$"
MSGC        DB    " channels",0Dh,0Ah,9,"$"
MSGD        DB    " samples per second (Hz)",0Dh,0Ah,9,"$"
MSGE        DB    " bits per channel",0Dh,0Ah,9,"$"
MSGF        DB    "samples are signed",0Dh,0Ah,0Dh,0Ah,"$"
MSGG        DB    "samples are unsigned",0Dh,0Ah,0Dh,0Ah,"$"
        ;
        ; Action flags.  1 = action selected.
        ;
MIXFLAG        DB    0
CONVERTFLAG    DB    0
HALVEFLAG    DB    0
        ;
        ; Prompts for the user.
        ;
MONOMSG        DB    "        Mix to mono?$"
CONVERTMSG    DB    "   Convert to 8-bit?$"
HALVEMSG    DB    "Halve sampling rate?$"
YHIMSG        DB    " (Y/n)  $"
NHIMSG        DB    " (y/N)  $"
        ;
        ; Buffer for user input.
        ;
USERBUF        DB    2
ANSWERED    DB    0            ; = 1 if the user answered
ANSWER        DB    2 DUP (0)        ; user's answer in first byte
DATA        ENDS

SCODE    SEGMENT
;
; Error handling subroutine.  Displays the string addressed by DS:DX and
; halts the program.
;
ERROR:
    MOV    AH,9
    INT    21h
    MOV    AX,4C01h        ; return ERRORLEVEL 1
    INT    21h
;
; Subroutine, takes pointer to string in DS:SI and length of string in CX,
; skips over blanks and tabs, returns pointer to first nonblank character
; in the string in DS:SI, length of remaining string in CX.  If end of
; string is reached, return pointer to end of string in DS:SI, zero in CX.
;
SKIPBLANKS:
    PUSH    AX
SKIPBLANKS_LOOP:
    JCXZ    SKIPBLANKS_END
    LODSB
    DEC    CX
    CMP    AL,9
    JE    SKIPBLANKS_LOOP
    CMP    AL,20h
    JE    SKIPBLANKS_LOOP
    DEC    SI
    INC    CX
SKIPBLANKS_END:
    POP    AX
    RET
;
; Subroutine, takes pointer to string is DS:SI and length of string in CX,
; skips over nonblank characters, returns pointer to first blank or tab in
; the string in DS:SI, length of remaining string in CX.  If end of string
; is reached, return pointer to end of string in DS:SI, zero in CX.
;
SKIPNONBLANK:
    PUSH    AX
SKIPNONBLANK_LOOP:
    JCXZ    SKIPNONBLANK_END
    LODSB
    DEC    CX
    CMP    AL,9
    JE    SKIPNONBLANK_LPEND
    CMP    AL,20h
    JNE    SKIPNONBLANK_LOOP
SKIPNONBLANK_LPEND:
    DEC    SI
    INC    CX
SKIPNONBLANK_END:
    POP    AX
    RET
;
; Subroutine, reads input and output filenames from the command line, opens
; the first file, and creates the second file.  Displays a usage message if
; two filenames are not specified.
;
OPENFILES:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    PUSH    DI
    ;
    ; Exchange DS, ES.  DS addresses PSP; ES addresses local data.
    ;
    PUSH    DS
    PUSH    ES
    POP    DS
    POP    ES
    ;
    ; Find first command line parameter, save pointer to it in DI.
    ;
    MOV    CL,[80h]
    XOR    CH,CH
    MOV    SI,81h
    CALL    SKIPBLANKS
    JCXZ    OPENFILES_USAGEERR
    MOV    DI,SI
    ;
    ; Find the end of it and save its length in BX.
    ;
    MOV    DX,SI            ; for Int 21h fcn 3Dh
    CALL    SKIPNONBLANK
    JCXZ    OPENFILES_USAGEERR
    MOV    BX,SI
    SUB    BX,DI
    ;
    ; Append a null.
    ;
    MOV    BYTE PTR [SI],0
    INC    SI
    DEC    CX
    JCXZ    OPENFILES_USAGEERR
    ;
    ; Open the input file.
    ;
    MOV    AX,3D20h        ; open read-only, deny writes
    INT    21h
    JC    OPENFILES_OPEN1ERR
    MOV    ES:INHANDLE,AX
    ;
    ; Find the second command line parameter.
    ;
    CALL    SKIPBLANKS
    JCXZ    OPENFILES_USAGEERR
    ;
    ; Append a null.
    ;
    MOV    DX,SI            ; for Int 21h fcn 3Ch
    CALL    SKIPNONBLANK
    MOV    BYTE PTR [SI],0
    ;
    ; Create the output file.
    ;
    MOV    AH,3Ch
    XOR    CX,CX            ; attribute:  normal file
    INT    21h
    JC    OPENFILES_OPEN2ERR
    MOV    ES:OUTHANDLE,AX
    ;
    ; Copy the input filename into the data segment.
    ;
    MOV    SI,DI            ; saved above
    MOV    CX,BX            ; also
    MOV    DI,OFFSET INFILENAME
    REP    MOVSB
    MOV    BYTE PTR ES:[DI],"$"
    ;
    ; Swap DS, ES back.
    ;
    PUSH    DS
    PUSH    ES
    POP    DS
    POP    ES
    ;
    ; Restore registers and exit.
    ;
    POP    DI
    POP    SI
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
    ;
    ; Two files were not specified on the command line.
    ;
OPENFILES_USAGEERR:
    PUSH    ES
    POP    DS
    MOV    DX,OFFSET USAGEMSG
    JMP    ERROR
    ;
    ; Unable to open input file.
    ;
OPENFILES_OPEN1ERR:
    PUSH    ES
    POP    DS
    MOV    DX,OFFSET INOPENMSG
    JMP    ERROR
    ;
    ; Unable to create output file.
    ;
OPENFILES_OPEN2ERR:
    PUSH    ES
    POP    DS
    MOV    DX,OFFSET OUTOPENMSG
    JMP    ERROR
;
; Subroutine to read a small number of bytes (specified in CX) from the input
; file into SMALLBUF.  Returns number of bytes in AX, pointer to SMALLBUF in
; DX.
;
READSMALL:
    PUSH    BX
    MOV    AH,3Fh
    MOV    BX,INHANDLE
    MOV    DX,OFFSET SMALLBUF
    INT    21h
    JC    READSMALL_READERR
    CMP    AX,CX
    JNE    READSMALL_INVALID
    POP    BX
    RET
    ;
    ; Error reading .wav file header.
    ;
READSMALL_READERR:
    MOV    DX,OFFSET READERRMSG
    JMP    ERROR
    ;
    ; Invalid .wav header, or unsupported .wav type.
    ;
READSMALL_INVALID:
    MOV    DX,OFFSET BADWAVMSG
    JMP    ERROR
;
; Subroutine for GETWAVHEADER.  Reads 4 bytes from the input file and checks
; whether the string read is "fmt ", "data", another ASCII string, or not
; ASCII.  Returns AL = 0 if "fmt ", AL = 1 if "data", AL = -1 if another
; ASCII string.  Halts the program if not ASCII.  Assumes ES addresses data
; segment.  Returns pointer to SMALLBUF in DX.
;
GETCHUNK:
    PUSH    BX
    PUSH    CX
    PUSH    SI
    PUSH    DI
    ;
    ; Read 4 bytes from the file.
    ;
    MOV    CX,4
    CALL    READSMALL
    ;
    ; Check if format chunk.
    ;
    MOV    SI,DX
    MOV    DI,OFFSET FMTSTR
    REPE    CMPSB
    JNE    GETCHUNK_CHKDATA
    MOV    AL,0
    JMP    GETCHUNK_EXIT
    ;
    ; Check if data chunk.
    ;
GETCHUNK_CHKDATA:
    MOV    SI,DX
    MOV    DI,OFFSET DATASTR
    MOV    CX,4
    REPE    CMPSB
    JNE    GETCHUNK_CHKASCII
    MOV    AL,1
    JMP    GETCHUNK_EXIT
    ;
    ; Check if a valid chunk header of another type.
    ;
GETCHUNK_CHKASCII:
    MOV    SI,DX
    MOV    CX,4
GETCHUNK_ASCLOOP:
    LODSB
    CMP    AL,20h
    JB    GETCHUNK_INVALID
    CMP    AL,7Eh
    JA    GETCHUNK_INVALID
    LOOP    GETCHUNK_ASCLOOP
    MOV    AL,-1
    ;
    ; Read the chunk length field and exit.
    ;
GETCHUNK_EXIT:    PUSH    AX
    MOV    CX,4
    CALL    READSMALL
    POP    AX
    POP    DI
    POP    SI
    POP    CX
    POP    BX
    RET
    ;
    ; Invalid .wav header, or unsupported .wav type.
    ;
GETCHUNK_INVALID:
    MOV    DX,OFFSET BADWAVMSG
    JMP    ERROR
;
; Subroutine for GETWAVHEADER.  This routine reads the format chunk from the
; input file and records the information found there.  Assumes DS:DX addresses
; the chunk length.
;
DOFORMAT:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    ;
    ; Check chunk length (must be 16).
    ;
    MOV    SI,DX
    LODSW
    CMP    AX,16
    JNE    DOFORMAT_INVALID
    MOV    CX,AX
    LODSW
    OR    AX,AX
    JNZ    DOFORMAT_INVALID
    ;
    ; Read in format chunk.
    ;
    CALL    READSMALL
    ;
    ; Verify format tag.
    ;
    MOV    SI,DX
    LODSW
    CMP    AX,1
    JNE    DOFORMAT_INVALID
    ;
    ; Get number of channels and save.
    ;
    LODSW
    OR    AX,AX            ; invalid if zero channels
    JZ    DOFORMAT_INVALID
    CMP    AX,16            ; an impossibly large value ...
    JA    DOFORMAT_INVALID
    MOV    INNCHANNELS,AX
    ;
    ; Get sampling rate.
    ;
    LODSW
    MOV    WORD PTR INSAMPRATE,AX
    MOV    DX,AX
    LODSW
    OR    DX,AX            ; rate of zero invalid
    OR    DX,DX
    JZ    DOFORMAT_INVALID
    MOV    WORD PTR INSAMPRATE+2,AX
    ;
    ; Get bits per channel.
    ;
    ADD    SI,6            ; skip bytes/sec, block align
    LODSW
    OR    AX,AX            ; zero bits per sample invalid
    JZ    DOFORMAT_INVALID
    CMP    AX,128            ; an impossibly large value ...
    JA    DOFORMAT_INVALID
    MOV    INSAMPWIDTH,AX
    ;
    ; Compute bytes per channel.
    ;
    ADD    AX,7
    SHR    AX,1
    SHR    AX,1
    SHR    AX,1
    MOV    INCHANSIZE,AX
    ;
    ; Compute bytes per (multichannel) sample.
    ;
    MUL    INNCHANNELS
    CMP    AX,16            ; this is what the buffer will hold ...
    JA    DOFORMAT_INVALID
    MOV    INSAMPSIZE,AX
    ;
    ; Are samples signed?
    ;
    CMP    INCHANSIZE,2
    CMC
    RCL    INISSIGNED,1
    ;
    ; Exit.
    ;
    POP    SI
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
    ;
    ; Invalid .wav header, or unsupported .wav type.
    ;
DOFORMAT_INVALID:
    MOV    DX,OFFSET BADWAVMSG
    CALL    ERROR
;
; Subroutine for GETWAVHEADER.  This routine skips over an unknown chunk,
; updating the file pointer.  Assumes DS:DX addresses the chunk length.
;
SKIPCHUNK:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    MOV    SI,DX
    LODSW
    MOV    DX,AX
    LODSW
    MOV    CX,AX
    MOV    AX,4201h
    MOV    BX,INHANDLE
    INT    21h
    POP    SI
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
;
; Routine to read the .wav header from the input file and compute needed
; parameters from it.  Skips unknown chunks.
;
GETWAVHEADER:
    PUSH    AX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    PUSH    DI
    PUSH    ES
    ;
    ; ES addresses data segment.
    ;
    MOV    AX,DS
    MOV    ES,AX
    ;
    ; Read RIFF and WAVE headers from file.
    ;
    MOV    CX,12
    CALL    READSMALL
    ;
    ; Verify "RIFF".
    ;
    MOV    SI,DX
    MOV    DI,OFFSET RIFFSTR
    MOV    CX,4
    REPE    CMPSB
    JNE    GETWAVHEADER_INVALID
    ;
    ; Verify "WAVE".
    ;
    ADD    SI,4            ; skip length field
    MOV    DI,OFFSET WAVESTR
    MOV    CX,4
    REPE    CMPSB
    JNE    GETWAVHEADER_INVALID
    ;
    ; Loop over chunks until data chunk found.
    ;
GETWAVHEADER_LOOP:
    CALL    GETCHUNK
    CMP    AL,0            ; format chunk?
    JNE    >L0
    CMP    FMTDONE,1        ; error if > 1 format chunk
    JE    GETWAVHEADER_INVALID
    CALL    DOFORMAT
    MOV    FMTDONE,1        ; mark format done
    JMP    GETWAVHEADER_LOOP
L0:    CMP    AL,1            ; data chunk?
    JNE    >L1
    CMP    FMTDONE,0        ; error if format chunk does
    JE    GETWAVHEADER_INVALID    ;   not precede data chunk
    JMP    GETWAVHEADER_LOOPEND
L1:    CALL    SKIPCHUNK        ; unknown chunk, skip
    JMP    GETWAVHEADER_LOOP
    ;
    ; Data chunk found, exit.
    ;
GETWAVHEADER_LOOPEND:
    POP    ES
    POP    DI
    POP    SI
    POP    DX
    POP    CX
    POP    AX
    RET
    ;
    ; Invalid .wav header, or unsupported .wav type.
    ;
GETWAVHEADER_INVALID:
    MOV    DX,OFFSET BADWAVMSG
    CALL    ERROR
;
; Subroutine to display a decimal number on the screen.  The number is in
; DX:AX.
;
SHOWDECIMAL:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    PUSH    DI
    MOV    BX,10            ; BX = 10 (constant for division)
    XOR    CX,CX            ; CX counts the digits
    MOV    DI,DX            ; DI:SI holds the number while it's
    MOV    SI,AX            ;   being divided away
    MOV    AX,DX
    ;
    ; Divide away the number, saving remainders on the stack.
    ;
SHOWDECIMAL_LOOP1:
    XOR    DX,DX
    DIV    BX
    MOV    DI,AX
    MOV    AX,SI
    DIV    BX
    MOV    SI,AX
    PUSH    DX            ; push digit (in low byte)
    INC    CX            ; bump count
    MOV    AX,DI            ; go again if not zero
    OR    AX,AX
    JNZ    SHOWDECIMAL_LOOP1
    OR    SI,SI
    JNZ    SHOWDECIMAL_LOOP1
    ;
    ; Pop the digits off the stack and display them.  (CX is now the loop
    ; counter.)
    ;
SHOWDECIMAL_LOOP2:
    POP    DX
    ADD    DL,'0'
    MOV    AH,2
    INT    21h
    LOOP    SHOWDECIMAL_LOOP2
    POP    DI
    POP    SI
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
;
; Routine to display information about the input .wav for the user.
;
SHOWWAVINFO:
    PUSH    AX
    PUSH    DX
    ;
    ; Display input filename.
    ;
    MOV    DX,OFFSET MSGA
    MOV    AH,9
    INT    21h
    MOV    DX,OFFSET INFILENAME
    MOV    AH,9
    INT    21h
    MOV    DX,OFFSET MSGB
    MOV    AH,9
    INT    21h
    ;
    ; Display number of channels.
    ;
    MOV    AX,INNCHANNELS
    XOR    DX,DX
    CALL    SHOWDECIMAL
    MOV    DX,OFFSET MSGC
    MOV    AH,9
    INT    21h
    ;
    ; Display sampling rate.
    ;
    MOV    AX,WORD PTR INSAMPRATE
    MOV    DX,WORD PTR INSAMPRATE+2
    CALL    SHOWDECIMAL
    MOV    DX,OFFSET MSGD
    MOV    AH,9
    INT    21h
    ;
    ; Display bits per channel.
    ;
    MOV    AX,INSAMPWIDTH
    XOR    DX,DX
    CALL    SHOWDECIMAL
    MOV    DX,OFFSET MSGE
    MOV    AH,9
    INT    21h
    ;
    ; Display whether samples are signed or unsigned.
    ;
    CMP    INISSIGNED,1
    JNE    >L0
    MOV    DX,OFFSET MSGF
    JMP    >L1
L0:
    MOV    DX,OFFSET MSGG
L1:
    MOV    AH,9
    INT    21h
    POP    DX
    POP    AX
    RET
;
; Subroutine to display a prompt to the user and get a "yes" or "no" answer.
; Takes 3 parameters.  DS:DX addresses the prompt to be displayed (terminated
; with a dollar sign).  DS:BX addresses the location where the answer will
; be stored (0 = no, 1 = yes).  AL is the default answer (if the user just
; hits return or enters something other than "y" or "n").
;
GETYN:
    PUSH    AX
    PUSH    CX
    PUSH    DX
    ;
    ; Save default answer in CL.
    ;
    MOV    CL,AL
    ;
    ; Display the prompt.
    ;
    MOV    AH,9
    INT    21h
    ;
    ; Display "(Y/n)" if "yes" is the default, or "(y/N)" if "no" is
    ; the default.
    ;
    CMP    CL,1
    JNE    >L0
    MOV    DX,OFFSET YHIMSG
    JMP    >L1
L0:
    MOV    DX,OFFSET NHIMSG
L1:
    MOV    AH,9
    INT    21h
    ;
    ; Get the answer.
    ;
    MOV    AH,0Ah
    MOV    DX,OFFSET USERBUF
    INT    21h
    ;
    ; Set result according to answer.
    ;
    MOV    [BX],CL                ; assume no (default) answer
    CMP    BYTE PTR ANSWERED,1        ; just exit if no answer
    JNE    GETYN_EXIT
    ;
    ; Get answer in AL and convert to 0 or 1.
    ;
    MOV    AL,ANSWER
    AND    AL,0DFh                ; convert to uppercase
    CMP    AL,'Y'
    JNE    >L0
    MOV    AL,1
    JMP    >L1
L0:
    CMP    AL,'N'
    JNE    GETYN_EXIT
    MOV    AL,0
L1:
    MOV    [BX],AL
    ;
    ; Move cursor to next line.
    ;
GETYN_EXIT:
    MOV    AH,2
    MOV    DL,0Dh
    INT    21h
    MOV    AH,2
    MOV    DL,0Ah
    INT    21h
    POP    DX
    POP    CX
    POP    AX
    RET
;
; Routine to prompt the user for the desired actions.  The user may choose
; to do one or more of the following:  mix to mono, convert to 8-bit, and/or
; cut the sampling rate in half.
;
GETACTIONS:
    PUSH    AX
    PUSH    BX
    PUSH    DX
    ;
    ; Ask if the user wants to mix to mono.
    ;
    MOV    DX,OFFSET MONOMSG
    MOV    BX,OFFSET MIXFLAG
    MOV    AL,1
    CALL    GETYN
    ;
    ; Ask if the user wants to convert to 8-bit.
    ;
    MOV    DX,OFFSET CONVERTMSG
    MOV    BX,OFFSET CONVERTFLAG
    MOV    AL,1
    CALL    GETYN
    ;
    ; Ask if the user wants to cut the sampling rate in half.
    ;
    MOV    DX,OFFSET HALVEMSG
    MOV    BX,OFFSET HALVEFLAG
    MOV    AL,0
    CALL    GETYN
    POP    DX
    POP    BX
    POP    AX
    RET
;
; Routine to fill in the fields in the output .wav header and write the
; header to the output file.
;
MAKEHEADER:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    PUSH    DI
    ;
    ; Determine number of channels in the output file.
    ;
    CMP    MIXFLAG,0
    JNE    >L0
    MOV    AX,INNCHANNELS            ; no mixing
    MOV    NCHANNELS,AX
    JMP    >L1
L0:
    MOV    NCHANNELS,1            ; mix to mono
    ;
    ; Determine sampling rate for output file.
    ;
L1:
    MOV    AX,WORD PTR INSAMPRATE
    MOV    DX,WORD PTR INSAMPRATE+2
    CMP    HALVEFLAG,0
    JE    >L2
    SHR    DX,1                ; cut sampling rate in half
    RCR    AX,1
L2:
    MOV    WORD PTR SAMPRATE,AX
    MOV    WORD PTR SAMPRATE+2,DX
    ;
    ; Determine number of bits per channel for the output file.
    ;
    CMP    CONVERTFLAG,0
    JNE    >L3
    MOV    AX,INSAMPWIDTH            ; do not convert to 8-bit
    MOV    SAMPWIDTH,AX
    MOV    AX,INCHANSIZE
    MOV    CHANSIZE,AX
    JMP    >L4
L3:
    MOV    SAMPWIDTH,8            ; convert to 8-bit
    MOV    AX,1
    MOV    CHANSIZE,AX
    ;
    ; Determine whether output samples are signed.
    ;
L4:
    CMP    AX,2
    CMC
    RCL    ISSIGNED,1
    ;
    ; Determine number of bytes per (multichannel) sample.
    ;
    MUL    NCHANNELS
    MOV    SAMPSIZE,AX
    ;
    ; Determine number of bytes per second.
    ;
    MOV    AX,WORD PTR SAMPRATE
    MUL    SAMPSIZE
    MOV    WORD PTR BYTESPERSEC,AX
    MOV    BX,DX
    MOV    AX,WORD PTR SAMPRATE+2
    MUL    SAMPSIZE
    ADD    AX,BX
    MOV    WORD PTR BYTESPERSEC+2,AX
    ;
    ; Determine number of (multichannel) samples from length of input
    ; file.
    ;
    MOV    AX,4201h        ; get current input file pointer
    MOV    BX,INHANDLE
    XOR    CX,CX
    XOR    DX,DX
    INT    21h
    JNC    >L5
    JMP    MAKEHEADER_READERR
L5:
    MOV    DI,DX            ; save in DI:SI
    MOV    SI,AX
    MOV    AX,4202h        ; seek to end of input file
    MOV    BX,INHANDLE
    XOR    CX,CX
    XOR    DX,DX
    INT    21h
    JNC    >L6
    JMP    MAKEHEADER_READERR
L6:
    SUB    AX,SI            ; determine number of sample bytes
    SBB    DX,DI
    MOV    BX,AX            ; divide high word by length of sample
    MOV    AX,DX
    XOR    DX,DX
    DIV    INSAMPSIZE
    MOV    WORD PTR INNSAMPLES+2,AX    ; save high word of result
    MOV    AX,BX                ; divide low word + remainder
    DIV    INSAMPSIZE
    MOV    WORD PTR INNSAMPLES,AX    ; save low word of result
    MOV    AX,4200h        ; seek back to start of sample data
    MOV    BX,INHANDLE
    MOV    CX,DI
    MOV    DX,SI
    INT    21h
    JC    MAKEHEADER_READERR
    ;
    ; Determine number of output samples.
    ;
    MOV    AX,WORD PTR INNSAMPLES
    MOV    DX,WORD PTR INNSAMPLES+2
    CMP    HALVEFLAG,0
    JE    >L7
    ADD    AX,1            ; divide by 2 and round up
    ADC    DX,0
    SHR    DX,1
    RCR    AX,1
L7:
    MOV    WORD PTR NSAMPLES,AX
    MOV    WORD PTR NSAMPLES+2,DX
    ;
    ; Determine length of output data.
    ;
    MUL    SAMPSIZE
    MOV    WORD PTR DATALEN,AX
    MOV    BX,DX
    MOV    AX,WORD PTR NSAMPLES+2
    MUL    SAMPSIZE
    ADD    AX,BX
    MOV    WORD PTR DATALEN+2,AX
    ;
    ; Determine length of output .wav file.
    ;
    MOV    DX,AX
    MOV    AX,WORD PTR DATALEN
    ADD    AX,36
    ADC    DX,0
    MOV    WORD PTR WAVLEN,AX
    MOV    WORD PTR WAVLEN+2,DX
    ;
    ; Write the header out.
    ;
    MOV    AH,40h
    MOV    BX,OUTHANDLE
    MOV    CX,44
    MOV    DX,WAVHEADER
    INT    21h
    JC    MAKEHEADER_WRITEERR
    POP    DI
    POP    SI
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
    ;
    ; Seek error on input file.
    ;
MAKEHEADER_READERR:
    MOV    DX,OFFSET READERRMSG
    JMP    ERROR
    ;
    ; Error writing to output file.
    ;
MAKEHEADER_WRITEERR:
    MOV    DX,OFFSET WRITEERRMSG
    JMP    ERROR
;
; Subroutine, reads data into input buffer, sufficient to make 1024 output
; samples.  The amount of data successfully read is returned in CX - the
; equivalent in output samples of the data successfully read.  Halts the
; program with an error message on file error.
;
READDATA:
    PUSH    AX
    PUSH    BX
    PUSH    DX
    ;
    ; Read from the input file.
    ;
    PUSH    DS
    MOV    AH,3Fh
    MOV    BX,INHANDLE
    MOV    CX,BYTESTOREAD
    MOV    DX,SEG INBUF
    MOV    DS,DX
    XOR    DX,DX
    INT    21h
    POP    DS
    JC    READDATA_READERR
    ;
    ; Check if the full amount requested was read.
    ;
    CMP    AX,CX
    JB    READDATA_EOF
    ;
    ; Got the full amount.
    ;
    MOV    CX,1024
    JMP    READDATA_EXIT
    ;
    ; End of file - less than the full amount requested was read.
    ;
READDATA_EOF:
    XOR    DX,DX            ; calculate number of samples read
    DIV    INSAMPSIZE
    MOV    CX,AX
    CMP    HALVEFLAG,0        ; if not halving, exit
    JE    READDATA_EXIT
    INC    CX            ; halving - divide by 2 and round up
    SHR    CX,1
    ;
    ; Exit.
    ;
READDATA_EXIT:
    POP    DX
    POP    BX
    POP    AX
    RET
    ;
    ; Error reading input file.
    ;
READDATA_READERR:
    MOV    DX,OFFSET READERRMSG
    JMP    ERROR
;
; Subroutine to write out the output buffer contents.  Halts the program
; with an error message if unsuccessful.  Uses BYTESPLACED.
;
WRITEDATA:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    DS
    MOV    AH,40h
    MOV    BX,OUTHANDLE
    MOV    CX,BYTESPLACED
    MOV    DX,SEG OUTBUF
    MOV    DS,DX
    XOR    DX,DX
    INT    21h
    POP    DS
    JC    WRITEDATA_WRITEERR
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
    ;
    ; Error writing to output file.
    ;
WRITEDATA_WRITEERR:
    MOV    DX,OFFSET WRITEERRMSG
    JMP    ERROR
;
; Subroutine to copy a multichannel sample from the input buffer to the small
; buffer in the data segment.
;
COPYSAMPLE:
    PUSH    AX
    PUSH    CX
    PUSH    SI
    PUSH    DI
    PUSH    DS
    PUSH    ES
    MOV    CX,INSAMPSIZE            ; CX = number of bytes to copy
    MOV    SI,BYTESTAKEN            ; DS:SI -> bytes to copy
    PUSH    DS
    POP    ES                ; ES:DI -> small buffer
    MOV    AX,SEG INBUF
    MOV    DS,AX
    MOV    DI,OFFSET SMALLBUF
    REP    MOVSB
    POP    ES
    POP    DS
    MOV    BYTESTAKEN,SI
    POP    DI
    POP    SI
    POP    CX
    POP    AX
    RET
;
; Subroutine to convert the multichannel sample in the small buffer to 8-bit.
;
CONVTO8:
    CMP    INCHANSIZE,1        ; if already 8-bit, return immediately
    JNE    CONVTO8_PROCEED
    RET
CONVTO8_PROCEED:
    PUSH    AX
    PUSH    CX
    PUSH    SI
    PUSH    DI
    PUSH    ES
    MOV    SI,OFFSET SMALLBUF
    PUSH    DS
    POP    ES
    MOV    DI,SI
    MOV    CX,INNCHANNELS
CONVTO8_CHANLOOP:
    ADD    SI,SKIPLENGTH
    LODSB
    ADD    AL,128
    STOSB
    LOOP    CONVTO8_CHANLOOP
    POP    ES
    POP    DI
    POP    SI
    POP    CX
    POP    AX
    RET
;
; Subroutine to mix a multichannel 8-bit unsigned sample.  The sample is
; assumed to be in the small buffer, and ES and DS are assumed to address
; the data segment.  The result is placed back in the small buffer.
;
MIX8:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    ;
    ; DS:SI addresses the sample.
    ;
    MOV    SI,OFFSET SMALLBUF
    ;
    ; Add up the channels.
    ;
    MOV    CX,INNCHANNELS
    XOR    BX,BX
MIX8_ADDLOOP:
    LODSB
    ADD    BL,AL
    ADC    BH,0
    LOOP    MIX8_ADDLOOP
    ;
    ; Divide by the number of input channels and save result.
    ;
    MOV    AX,BX
    XOR    DX,DX
    DIV    INNCHANNELS
    MOV    SMALLBUF,AL
    POP    SI
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
;
; Subroutine to mix a multichannel 16 (or more)-bit signed sample.  The
; sample is assumed to be in the small buffer, and ES and DS are assumed
; to address the data segment.  The result is placed back in the small
; buffer.
;
MIX16:
    PUSH    AX
    PUSH    BX
    PUSH    CX
    PUSH    DX
    PUSH    SI
    PUSH    DI
    ;
    ; Clear the mixing buffer.
    ;
    MOV    DI,OFFSET MIXBUF
    MOV    AL,0
    MOV    CX,CHANSIZE
    REP    STOSB
    STOSB
    ;
    ; Loop over the channels.
    ;
    MOV    SI,OFFSET SMALLBUF
    MOV    CX,INNCHANNELS
MIX16_ADDLOOP:
    PUSH    CX
    ;
    ; Copy all but the last byte of the channel to the channel buffer.
    ;
    MOV    CX,CHANSIZE        ; CHANSIZE is at least 2 for signed
    DEC    CX
    MOV    DI,OFFSET CHANBUF
    REP    MOVSB
    ;
    ; Get the last byte, sign extend, add to convert to unsigned, and
    ; place in the channel buffer.
    ;
    LODSB
    CBW
    ADD    AX,80h
    MOV    [DI],AX
    ;
    ; Add the channel to the mixing buffer.
    ;
    PUSH    SI
    MOV    SI,OFFSET CHANBUF
    MOV    DI,OFFSET MIXBUF
    MOV    CX,CHANSIZE
    INC    CX
    CLC
    LAHF
MIX16_DOADD:
    LODSB
    SAHF
    ADC    [DI],AL
    LAHF
    INC    DI
    LOOP    MIX16_DOADD
    POP    SI
    ;
    ; Go to next channel.
    ;
    POP    CX
    LOOP    MIX16_ADDLOOP
    ;
    ; Divide by the number of channels.
    ;
    STD                ; set direction to decrement
    MOV    CX,CHANSIZE
    MOV    SI,CX
    ADD    SI,OFFSET MIXBUF
    DEC    CX
    MOV    DI,CX
    ADD    DI,OFFSET SMALLBUF
    ;
    ; Divide high 2 bytes as word to avoid overflow.
    ;
    LODSB
    MOV    AH,AL
    LODSB
    XOR    DX,DX
    DIV    INNCHANNELS
    STOSB                ; discard high 8 bits of quotient
    MOV    AH,DL
    ;
    ; Complete the division.
    ;
    MOV    BL,BYTE PTR INNCHANNELS
MIX16_DIVLOOP:
    LODSB
    DIV    BL
    STOSB
    LOOP    MIX16_DIVLOOP
    CLD                ; restore direction flag
    ;
    ; Convert the result to signed.
    ;
    MOV    SI,OFFSET SMALLBUF-1
    ADD    SI,CHANSIZE
    ADD    BYTE PTR [SI],80h
    POP    DI
    POP    SI
    POP    DX
    POP    CX
    POP    BX
    POP    AX
    RET
;
; Subroutine to mix the multichannel sample in the small buffer to mono.
; Assumes that the sample has already been converted to 8-bit if that was
; to be done.  This version special-cases 8-bit samples.
;
; Exit immediately if already mono.
;
DOMIX:
    CMP    INNCHANNELS,1
    JNE    DOMIX_PROCEED
    RET
    ;
    ; Both DS and ES address the data segment.
    ;
DOMIX_PROCEED:
    PUSH    ES
    PUSH    DS
    POP    ES
    ;
    ; Call the appropriate subroutine for signed or unsigned (8-bit)
    ; samples.
    ;
    CMP    ISSIGNED,0
    JE    DOMIX_8BIT
    CALL    MIX16
    JMP    DOMIX_EXIT
DOMIX_8BIT:
    CALL    MIX8
DOMIX_EXIT:
    POP    ES
    RET
;
; Subroutine to copy a sample from the small buffer to the output buffer.
; Uses and updates BYTESPLACED.
;
PUTSAMPLE:
    PUSH    CX
    PUSH    SI
    PUSH    DI
    PUSH    ES
    MOV    CX,SEG OUTBUF
    MOV    ES,CX
    MOV    DI,BYTESPLACED
    MOV    SI,OFFSET SMALLBUF
    MOV    CX,SAMPSIZE
    REP    MOVSB
    MOV    BYTESPLACED,DI
    POP    ES
    POP    DI
    POP    SI
    POP    CX
    RET
;
; Routine to perform the actual conversion of samples in the input file and
; write the results to the output file.  Samples are converted 1024 output
; samples at a time; if HALVEFLAG is on, this will be 2048 input samples.
;
PERFORM:
    PUSH    AX
    ;
    ; Determine number of input samples for each pass.
    ;
    CMP    HALVEFLAG,0
    JE    PERFORM_1024
    MOV    SAMPTOREAD,2048
PERFORM_1024:
    ;
    ; Determine number of input bytes for each pass.
    ;
    MOV    AX,SAMPTOREAD
    MUL    INSAMPSIZE
    MOV    BYTESTOREAD,AX
    ;
    ; Determine number of bytes to "skip over" from each input channel.
    ;
    MOV    AX,INCHANSIZE
    SUB    AX,CHANSIZE
    MOV    SKIPLENGTH,AX
    ;
    ; Main loop.  Read data (BYTESTOREAD bytes) from the input file into
    ; the input buffer.
    ;
PERFORM_MAINLOOP:
    CALL    READDATA
    JCXZ    PERFORM_MAINLPEND        ; exit loop and end of file
    ;
    ; Number of bytes taken from the input buffer set to zero.
    ;
    MOV    BYTESTAKEN,0
    ;
    ; Number of bytes placed in output buffer set to zero.
    ;
    MOV    BYTESPLACED,0
    ;
    ; Loop over samples in the buffer.
    ;
PERFORM_SAMPLOOP:
    CALL    COPYSAMPLE        ; get a sample in the small buffer
    ;
    ; If converting to 8-bit, do the conversion.
    ;
    CMP    CONVERTFLAG,0
    JE    PERFORM_CHKMIX
    CALL    CONVTO8
    ;
    ; If mixing to mono, do so.
    ;
PERFORM_CHKMIX:
    CMP    MIXFLAG,0
    JE    PERFORM_DOPUT
    CALL    DOMIX
    ;
    ; Put the sample in the output buffer.
    ;
PERFORM_DOPUT:
    CALL    PUTSAMPLE
    ;
    ; If halving the sampling rate, skip a sample.
    ;
    CMP    HALVEFLAG,0
    JE    PERFORM_SAMPLPTEST
    MOV    AX,BYTESTAKEN
    ADD    AX,INSAMPSIZE
    MOV    BYTESTAKEN,AX
PERFORM_SAMPLPTEST:
    LOOP    PERFORM_SAMPLOOP
    ;
    ; Write the output buffer and go read more data.
    ;
    CALL    WRITEDATA
    JMP    PERFORM_MAINLOOP
PERFORM_MAINLPEND:
    POP    AX
    RET
;
; Main program.
;
MAIN:
    ;
    ; DS addresses data segment (always - if any routine changes DS, it
    ; must change it back before returning).
    ;
    MOV    AX,SEG SDATA
    MOV    DS,AX
    ;
    ; Set direction to increment (always - if any routine changes DF, it
    ; must restore it before returning).
    ;
    CLD
    ;
    ; Open input file and create output file.
    ;
    CALL    OPENFILES
    ;
    ; Read the .wav header from the input file.
    ;
    CALL    GETWAVHEADER
    ;
    ; Display the header information for the user.
    ;
    CALL    SHOWWAVINFO
    ;
    ; Get desired actions from user.
    ;
    CALL    GETACTIONS
    ;
    ; Fill in fields in the output .wav header and write it out.
    ;
    CALL    MAKEHEADER
    ;
    ; Convert the file.
    ;
    CALL    PERFORM
    ;
    ; Terminate.
    ;
    MOV    AX,4C00h
    INT    21h
SCODE    ENDS
    END    MAIN