;--------------------------------------------------------------------------
;
;   Program:   dcomp
;   Author:    Ty Devries
;              Clint Fitch
;   Date:      26 October, 2011
;
;   Function:  Dcomp decompresses ASCII text.
;
;   Input:
;   - si points to the string of compressed data
;   - di points to the empty list into which the decompressed data is stored
;
;   Output:
;
;   - The compressed data in inlist is decompressed into output list
;   - The compressed data is not modified
;   - All registers contain their original value except ax
;     ax = 0        Invalid compressed bit pattern found
;     ax = 1...n    Okay and n = size of decompressed data
;
;--------------------------------------------------------------------------


;----------------------------------
         .model    small          ;64k code and 64k data
         .8086                    ;only allow 8086 instructions
         public    dcomp          ;allow linker to access  dcomp
         public    getbit         ;allow linker to access  getbit
;----------------------------------


        .data
;-------------------------------------------------------------
;
;-------------------------------------------------------------


         .code
;----------------------------------
;
;----------------------------------
dcomp:
                                  ;
         ret                      ; return
;---------------------------------;


         .data
;----------------------------------
;
;----------------------------------
eight   db  8         ; mothafuckin 8
;----------------------------------
; GETBIT
;
; Input:
; - ax contains  the bit number requested ( 0 to 1783 are valid)
; - si points to the input compressed data string 1 to 223 bytes
;
; Output
; - if the requested bit number is valid (0 to 1783) then
;   ax contains the bit value 0 - 1
; - if the requested bit number is invalid  (greater than 1783)  then
;   ax is unchanged
; - the subroutine must save and restore any other used register
; - the input compressed data may not be changed
;----------------------------------
         .code                    ;
;----------------------------------
; Prepare bit to be got
;----------------------------------
getbit:                           ;
     cmp    ax,1783       ; compare ax to 1783
     ja return        ; if ax > 1783, return
     push   bx        ; store bx
     push   cx        ; store cx
     div    [eight]       ; ah = remainder, al = quotient
     mov    bl,al         ; bl is byte offset
     mov    bh,0          ; convert bl to word
     mov    al,[bx+si]    ; looking at correct byte
         mov    cl,ah         ; cl is bit offset
     inc    cl        ; always loop once more
;----------------------------------
; Shift bitwise to the desired bit
;----------------------------------
shift:                ;
     shl    al,cl         ; shift left once
;----------------------------------
; Move the registers back to their
; former values
;----------------------------------
restore:              ;
     pop    cx        ; restore cx
     pop    bx        ; restore bx
;----------------------------------
; Set ax to 1 if CF is 1
;----------------------------------
getflag:              ;
     jnc    zero          ; check the carry flag
     mov    ax,1          ; bit is a 1
     ret              ; return to main
;----------------------------------
; Otherwise set ax to 0
;----------------------------------
zero:                 ;
     mov    ax,0          ; bit is a 0
;----------------------------------
; Exit subroutine and return to
; main
;----------------------------------
return:               ;
         ret                      ; return bit
;----------------------------------
         end