HPAVC/HPAVC CD-ROM.iso/pc/ASMVEG5.ZIP/ASMVEG5.TXT

Text File | 1996-12-23 | 16.6 KB | 482 lines

      Assembly Language for Veggies (And C programmers)      Part 5.

Welcome to 1992! It is now several months since the last ASMVEG series...  In
this time I have been learning a bit about Graphics on the PC, and mucking
about with Turbo Pascal, however it has now come time to again put words
onscreen about dear old Assembler once more...

Last issue, we looked at The TSR, and started mentioning some tricky things
that are important to programming TSRs...  This issue i'd like to delve into
some simpler work by presenting some simple routines for doing "Workhorse"
stuff - ya know, stuff you take for granted like screen clearing and file
reading/writing etc etc which everyone always seems to assume you know..

Oh, if you have a WordProcessor there with adjustable tabstops, set them to 20
spaces between tabs or the listings will look shitty!

OK, here goes...


Screen Clear
------------

A simple routine for a simple job: Clears the screen in the current screen
mode. Homes cursor on text screens, homes ligical cursor on graphics screens.

Clear_screen:         push ax
         mov ah,0fh
         int 010h
         mov ah,0
         int 010h
         pop ax
         ret

Very easy! Does all the usual jazz like resetting the colours to their
defaults etc. This is a standard sort of thing that I use in just about every
program I can ever remember writing...  it works, it's reliable, and let's
face it, who wants to clear the screen faster??


Go "Beep"
---------

Yet another rediculous task. Make a beep! This beep is the standard PC beep,
such as you get when you type a text file full of CTRL-G's to the screen or
fill uo your keyboard buffer, and is instantly recognisable as the work of the
BIOS..

Go_Beep:         push ax
         push bx
         mov ah,0eh
         mov al,07
         mov bh,0
         int 010h
         pop bx
         pop ax
         ret

Call this as often as you like, but remember that your whole program is paused
for the length of the tone.


What Video?
-----------

Ever get sick of asking users their video card type? Why not auto-detect it
yourself and TELL them what you detected...  simple as pie! These video bios
functions were built in to assist the programmer and take the hassle of asking
the user what his card is.. buggered if I know why game writers don't just use
this stuff themselves.... maybe they're stupid C programmers?

Detect_VGA:         mov ax,01a00h
         int 010h
         cmp al,01ah

         ( if AL=0a1h, then VGA is present )

If you have a MCGA (PS/2) system, this will detect the MCGA. To differentiate
between MCGA and true VGA is a difficult thing..  if you specificly want to do
something other than 320x200x256 mode then you'll be needing to detect a SVGA
anyhow, so it doesn't really matter.

Detect_EGA:         mov ah,012h
         mov bl,010h
         int 010h
         cmp bl,010h

         ( if BL=010h then EGA is NOT present )

NB: if BL is not 010h, then it will indicate this:

 BL = 0,  64k EGA RAM
 BL = 1, 128k EGA RAM
 BL = 2, 192k EGA RAM
 BL = 3, 256k EGA RAM

WARNING: Use this AFTER Detect_VGA because a VGA will also return this
information as well (After all, a vGA emulates EGA don't it?!?!)

OK..  if it's not EGA or VGA then it MUST be either CGA or MONO ... if it's a
mone, the screen MUST be in mode 7, if it's CGA it will be in another mode..

Detect_CGA_MONO:         mov ah,0fh
         int 010h
         cmp al,7

         ( if AL=7, MONO is present, else must be CGA )

Furthermore: if BOTH a MONO and ANY colour card are present, and you want to
make sure, first detect the Default card...  if you find a colour card, try
calling setvideomode and try setting mode 7. Next, read the mode back. if
you're in mode 7, MONO is present. if not, then you only got colour. If you
detect MONO as default and you don't find VGA or EGA, try setting mode 3 to
test for CGA. If you can set mode 3 then CGA is present, but not default.


KeyPressed?
-----------

Moving right along....  the BIOS has some good keypress routines indeed,
in fact I have never needed to do anything keyboardish and not been able to
use the BIOS for it...  oh, and the BIOS is very quick with the keyboard too -
definately suitable for use where high speed is required.

Is_A_Key_Pressed:         mov ah,1
         int 016

If the Z (Zero) flag is set, then NO keys have been pressed. If the Z flag is
NOT set a key has been pressed. This does not retrieve a key from the keyboard
buffer, it just lets you know there's one there to be read later on.


Next Key in Buffer
------------------

Retrieve_keypress:         mov ah,0
         int 016

This gets the next key that has been pressed and puts the "Main byte" in AL,
and the "Auxiliary Byte" in AH. normally AL holds the ASCII code for the key
pressed, but if the key is not a normal key (like a function or arrow key)
then AL=0 and AH holds the so called EXTENDED keycode... tables of extended
keycodes should be in any good ASM book - Norton's has it for one. Note, if
you call this service BEFORE a key is ready, the routine pauses till one key
is hit, which freezes the program in effect...  saves a look for key, until
pressed, then read key loop though.


Keyboard Lights (AT Computers ONLY!)
------------------------------------

Ever wanted to stuff with the keyboard lights? Once again the BIOS will help
us there... At absolute memory location 0040:0017 - that's right - the BIOS
data table...

Here is the layout of that byte:

  bit#    7      6    5     4      3    2      1      0
       Insert  Caps  Num  Scroll  ALT  CTRL  L.SHI  R.SHI

Altering any of these bits will alter the computer's idea of what's what...
changing bits 6-4 will toggle the 3 lights on the KB.. all courtesy of the
BIOS.. neat and quick...  Fiddling with INS changes the INS state, but mucking
with alt, ctrl and the shifts is not on, otherwise you REVERSE the action of
that key; held down becomes like released etc...  if you turned them all
opposite to what they normally are, then to type normally one would have to
hold down ALT, CTRL and both shift keys!!


Reading a file to memory
------------------------

One of the hardest routines to master is file access. Here it is simplified.

Filename         db 'filexxxx.!!!',0
Handle         dw 0
memseg         dw 07000h

Load_File:         mov ah,03dh
         mov al,0
         mov dx,offset filename
         int 021h

This section opens the file for Read/Write access... changing AL changes the
access thus:

   AL = 0    Read/Write
   AL = 1    Write Only
   AL = 2    Read Only

Obviously the filename is held in an ASCIIZ string at Filename. ASCIIZ stands
for ASCII with a 0 on the end basicly..  the 0 tells DOS where the end of the
string is.

         jc FAIL

If carry is set on return, the open operation FAILED. The file must exist and
be able to be opened for this operation to work. An error code will be passed
in AX - again see Norton's or other for errorcode listings.

         mov handle,ax

DOS has what is called a handle passed into AX if the open was successfull..
each file has a unique handle, and there may be up to the number of files
liste din the FILES= statement in Config.sys open at any one time. note that
StdIn, StdOut, StdErr and Null are always open by DOS and are used when
"piping" program outpout to and from files. (DIR > files.lst for example) so
normally the handle for the fist open file will be 0005. We must store
whatever we get back in any case so this is done.

At this stage we have simply requested DOS to get us a file. We haven't read
it yet.

read_from_file:         mov dx,word ptr cs:[memseg]
         push dx
         pop ds
         mov dx,0
         mov ah,03fh
         mov bx,word ptr cs:[handle]
         mov cx,0ffffh
         int 021h
         jc FAIL
         cmp ax,0ffffh
         jnz end_read

         mov dx,word ptr cs:[memseg]
         add dx,01000h
         mov word ptr cs:[memseg],dx
         jmp read_from_file

end_read:         push cs
         pop ds

Now this is quite complex and takes a while to understand.... read up on how
the registers are bieng used... basicly the file is loaded in 64k chunks
(Because that's the biggest DOS can do in one go) to the 64k chunk of memory
located at DS:DX. This program happens to hard code the load address at 07000h
but normally you'd change that to the place you'd requested from DOS. It also
assumes to start at offset 0 in that segment...  change DX if you start part
way into the segment. The routine attempts to load 64k; and will load up to
the limit of a less-than-64k file. Again, C is set if there is an error same
as the open routine. Next, AX holds the bytes read. if AX is less than 0ffffh
then less than 64k was read and we're at the end of the file. If so, jump out.
If not, we increment the memory segment and jump out.

Note that DS has to be changed to suit the routine, thus the use of CS:
override... the CS override makes sure the data we're storing does in the
right segment (remember that normally DS=CS, but since we're changing DS, we
must override the normal use of DS and make it use CS so it looks in the right
segment)

At this stage our file is loaded and memseg holds the final segment of the
file, whilst ax holds the number of bytes in the last segment. simple
subtraction will provide the full file size thus:

Loaded file size (in bytes)   =    ((memseg-inital segment)*64k)+ax

NB: the final number will be a double word - 32 bits long, so don;t try and
load it into a register, or even work it out this way!!

After we are finished with the file we must tell DOS so. We must call the
Close File function to tell dos to return the Handle to the unused pool and
also to save any changes we may have written to the file physicly to disk.
ALWAYS CLOSE THE FILE!!!  Even if you get errors, CLOSE THAT FILE!! This is
the ONLY way to avoid your program making chop suey of the disk drive's files!!

Close_File:         mov ah,03eh
         mov bx,handle
         int 021h

The file is now closed. Again Carry reports an error, but the only possible
error is ax=6 which is invalid handle (Caused by feeding it a handle number
that is not in use!) otherwise AX = 0 on exit.



Writing Files to disk
---------------------

Writing a file is identical to reading it...  Open the file, write the data,
then close it... just use the different functions as listed in your ASM book.
Use function 3d to write an existing file. Function 3c is used to write a new,
non existant file, or to completely trash the contents of an existing file.


I/O ports present
-----------------

What Serial and Parallel ports are there?  Memory locations 0040:0000 thru
0040:000F hold the addresses of the first four Serial ad parallel ports viz:


0040:0000 | f8 03 | f8 02 | e8 03 | e8 02 | bc 03 | 78 03 | 78 02 | 00 00 |
          | COM1: | COM2: | COM3: | COM4: | LPT1: | LPT2: | LPT3: | LPT4: |

So, for example, to find the address of LPT1:, simply code:

         push ds
         mov ax,040h
         push ax
         pop ds
         mov dx,word ptr[8]
         pop ds

DX now holds the address of LPT1:.  You should remember that these ports are
16 bit addresses, so use a 16 bit register, and also remember that in PC's the
Most signivigant bit is stored second.

If you find 00 00 stored then there are no more ports to be found - in the
example above for example there are but 3 parallel ports. This information is
placed by the BIOS so once again it is quite reliable, and also multitasking
friendly to read these bytes. The only side effect is that sometimes
non-standard serial ports will not show up here (ie serial ports at non
standard addresses) - although drivers that usually come with these ports
usually update this area if needed. BBS Fossil drivers are one such driver
that will, for example.


Reading AT CMOS Settings
------------------------

Everyone wants to know this!!

CMOS_INFO         db 03fh dup(?)

Read_CMOS:         push ds
         pop es
         mov cx,03fh
         mov bl,00h
         mov di,offset CMOS_INFO
         cli

read_in:         mov dx,0070h
         mov al,bl
         out dx,al
         mov dx,0071h
         in al,dx
         stosb
         inc bl
         loop read_in
         sti

This routine basicly outputs a byte reprsenting the byte in the CMOS we want
to read to port 0070h, then reads in the byte from port 0071h. It does this
03fh times to read the entire CMOS to memory.

The Layout of the CMOS is thus:

byte#         Funtion

Real Time Clock Bytes

  0         Current Second in BCD format
  1         Second Alarm in BCD
  2         Current Minuute in BCD
  3         Minute alarm in BCD
  4         Current hour in BCD
  5         Hour alarm in BCD
  6         Current day of week in BCD
  7         Current date in BCD
  8         Current month in BCD
  9         Current year in BCD

Status bytes

  a,b,c,d         Status bytes A,B,C & D

System configuration

  e         Diagnostic Status
  f         ShutDown Reason
 10         Diskette drive types where:
         bit 7-4 drive 0 (A:)  bit 3-0 drive 1 (B:) where
           0000b = none
           0001b = 360k
           0010b = 1.2M
           0011b = 720k
           0100b = 1.44M

 11         fixed disk drive 0 type (0 = none, 1-46 = type)
 12         fixed disk drive 1 type (0 = none, 1-46 = type)

 13         reserved

 14         equipment byte where:
         bit 7-6 = number of drives, where 00b = 1, 01b = 2
         bit 5-4 = display, where:
           00b = reserved
           01b = cga 40 cols
           10b = cga 80 cols
           11b = cga in mono mode
         bit 3-2 reserved
         bit 1 = math co present
         bit 0 = diskette drive present

 15         Base memory in k, low byte
 16         Base memory in k, high byte
 17         Extended memory in k, low byte
 18         Extended memory in k, high byte

 19         disk type of first fixed disk
 1a         disk type of second fixed disk

 1b-2dh         reserved

 2e         high byte checksum of bytes 10-2dh
 2f         low byte checksum of bytes 10-2dh

 30         Low byte actual Extended memory size
 31         high byte actual Extended memory size

 32         century in BCD

 33         information flag

 34-3f         reserved

This list is based on an early phoenix BIOS. Newer bios's wil have the
reserved areas changed, and any bytes after 2fh are fair game.. Use this info
at your own peril because ther is no guarantee of manufacturer compatability -
Compaq and amstrad bieng but two known misconformers. All BIOS's with "Type
47" user Hard Drive types will undoubtedly be non-standard too.

BCD Refers to binary coded decimal number format. This format is made to store
the numbers 0-9 only. Imagine hex, but just omit the letters.. you got BCD..

Viz:   8d = 00001000 bcd
      11d = 00101000 bcd
      99d = 10011001 bcd
      47d = 01000111 bcd

Not as efficient bit-wize as binary (the biggest 8 bit number in BCD is 99)
but simple to decode - split into high and low nibbles and treat as binary.


FastWrite String to Screen
--------------------------

Sick of DOS int 21h function 09h write string? Try this one - it's a BIOS
string writer that's about twice as fast as DOS.. you may find it worth using
for although it's longer than dear old dos into 21, it's surely miles the
quicker! It is designed as a "drop in" replacement for dos int 21, function
09, and takes the same setup, ie $ terminated string pointed to by DS:DX. All
registers are preserved in this routine.


write_bios_string:         push ax
         push bx
         push si
         mov si,dx

loop_write:         lodsb
         cmp al,'$'
         jz exit_write

         mov ah,0eh
         mov bx,0
         int 010h
         jmp loop_write

exit_write:         pop si
         pop bx
         pop ax
         ret


------------------------------------------------------------------------------

Err...  well that kinda brings to a close this ASMVEG...  I've brought to
light a few routines I was meaning to hand out long ago, and added a couple
more I just thought of :-)

Next time round i'll probably have some more code in runnable form, and will
be looking at a specific program .. i'm not sure exactly what yet :-) but it
will be coming... Perhaps we'll look at making musical sounds and perhaps we
will look at low level disk structures or something...  i'm not exactly sure
yet....

Anyhow, until the next VEG, keep hacking!

                           .\\erlin