Bsave

 BBS: Inland Empire Archive
Date: 03-11-92 (01:11)             Number: 78
From: JEAN CREPEAU                 Refer#: NONE
  To: TIM LAWING                    Recvd: NO  
Subj: Bsave                          Conf: (2) Quik_Bas

In a message to ALL, TIM LAWING wrote:
TL=> I have a screen that I  want to save. I have tried to  understand this
    from a couple different manuals, still not clear. The screen is  Screen
    0. Someone please explain to me  the process of reading the screen  and
    placing it into a file. Please explain Def Seg to thanks.

        The video  image is  stored somewhere  in memory  of your computer.
This area of the memory is usually called video RAM. First of all, you must
understand how the memory works and how your micro-processor can access it.

        The  memory  is  divided  into  'cells'.  Each cell has a different
adress and  can hold  a byte.  A byte  is a  collection of  8 binary digits
(bits)... Thus  each cell  can hold  a value  between 00000000  to 11111111
(binary) or 0 to 255 (decimal).

        With the old 8086, the CPU (micro-processor) can access up to  1Meg
(2^20 =  1 048  576) bytes  of memory.  Then you  need 1  048 576 different
adresses to access each of these bytes.  A full adress needs 20 bits to  be
represented.  Physically,  that  means  that  your  micro-processor  has 20
electrical nets  that tells  the memory  "Hey! I  want to  know what is the
content of that cell".  The memory then returns  the specified byte to  the
CPU.

        In the 8086, you have  no registers (hardware variable) of  20 bits
that could hold a complete adress. Intel had to use an indirect way to  get
these adresses of 20 bits with the registers available. Usually a  register
is 8 or  16 bits wide.  So Intel took  two registers of  16 bits (a segment
register and an index (offset) register. The final adress was computed with
the following formula:

                Adress = Segment * 16 + Offset

        Usually,  we  do  not  specify  adresses  (segments  or offsets) in
decimal... Binary is more direct, but it  is much too long to write 20  0's
or 1's every  time we need  to specify an  adress!!! So we  use hexadecimal
that is closely related to binary. Bits are grouped four-by-four to form  a
single hexadecimal (base 16) digit. This is the translation table bin2hex:
        0000 - 0        0100 - 4        1000 - 8        1100 - C
        0001 - 1        0101 - 5        1001 - 9        1101 - D
        0010 - 2        0110 - 6        1010 - A        1110 - E
        0011 - 3        0111 - 7        1011 - B        1111 - F

        If we write the adress formula into hexadecimal, we have:

                Adress = Segment * 10h + Offset

        10h means 10 in hexadecimal and  it is equal to 00010000 in  binary
and 16 in decimal. Now! The video RAM is always located at the same  adress
in memory. This adress is
                B0000h for monochrome cards
                B8000h for color cards

        To translate these adress into segment and offset, a simple  simple
way to do so, is to take the 4 first digits as the segment and the last one
as  the  offset.  Then  B0000h  =  B000h:0000h and B8000h = B800h:0000h. We
usually  write  an  adress  either  in  the  full  5-digit form or with the
segment:offset form, which is more explicit.

        Now you know everything about segments! When you want to do a BSAVE
or BLOAD,  you must  specify in  which segment  these commands  take place.
Since it is to be used with your video RAM, you either do
        DEF SEG = &hB000        ' for mono cards
or      DEF SEG = &hB800        ' for color cards

        With  BLOAD/BSAVE,  you  cannot  specify  full adress, you can only
specify the  offsets. All  the adresses  have to  be in  the same  segment.
Screen 0 is at the offset 0 (0000h). The syntax for BLOAD/BSAVE are:

BLOAD filename$ [,offset]
        This command loads  a command saved  with BSAVE. If  you specify no
offset, the segment and offset saved into the file are used. If you specify
offset, the  file is  loaded at  the adress  specified by  the last DEF SEG
definition and the offset.

BSAVE filename$,offset,lenght
        This  commands  save  the  memory  area  beginning  with the adress
specified  by  the  last  DEF  SEG  and  the offset for the number of BYTES
specified.

        To save screen 0,  you must save 4000  bytes (80*25*2 = 4000)  from
the adress B000:0 or B800:0.

For examples:
DEF SEG = &hB800        ' defines segment of video RAM (color)
BSAVE "SCREEN0",0,4000  ' saves screen 0 to file "SCREEN0"

DEF SEG = &hB000        ' defines segment of video RAM (monochrome)
BLOAD "SCREEN0"         '  loads  SCREEN0  at  the  adresse  saved by BSAVE
                                (B800:0)
BLOAD "SCREEN0",0       '  loads  SCREEN0  at  the  adresse  B000:0.  It is
                                necessay  to  specify  the  offset 0 if you
                                want QuickBASIC to use the correct segment.

DEF SEG = &hB800        ' def video RAM seg (color)
BLOAD "SCREEN0"         ' loads SCREEN0 into screen 0
BLOAD "SCREEN0",&h1000  ' loads SCREEN0 into screen 1
BLOAD "SCREEN0",&h2000  ' loads SCREEN0 into screen 2
BLOAD "SCREEN0",&h3000  ' loads SCREEN0 into screen 3

        Hope it helps you to understand! Jean
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