Think #47 30 августа 2000

Программирование

Programmers - Displays a three-character file extensions.

 Derivation of three-character file extensions

      in the operating system TR-DOS



                  (C) Ivan Roshchin, Moscow


Fido: 2:5020 / 689.53
E-mail: asder_ffc@softhome.net
WWW: http://www.zx.ru/echo/roschin



    In the TR-DOS each file is put in
compliance with the 16-byte descriptor that is in your drive. 
As seen from tables in this descriptor for expansion

file by only one symbol:


 Shift
 The length from the beginning Comment


     0 8 Filename


     August 1 File Extension


     September 2 starting address file

                   (For BASIC-length of
                   programs and variables)


    February 11 Length of file in bytes

                   (For BASIC-length of
                   programs without regard to length
                   Ny variables)


    January 13 length of the file in the sector
                   tures


    January 14 start sector number

                   File


    January 15 Track number beginning

                   File



    But in recent years increasingly used a more informative 
three-letter file extension (as in MS-DOS):




  Shift
 The length from the beginning Comment


     0 8 Filename


     August 3 File Extension


     February 11 Length of file in bytes

                   (For BASIC - the length of

                   program without

                   length variables)


     January 13 length of the file in the sector
                   tures


     January 14 start sector number

                   File


     January 15 Track number beginning

                   File



    It is seen that the length of the extension due to the use 
of two bytes, which had been stored starting address

file (for BASIC-files - the length of the program
and variables).


    If you write a program
for working with files, it would be nice to include in its 
three-character output extensions. But here comes the problem:

not known in advance which files will have to deal, and the 
extension of files may be a three-character and 
single-character. Thus, for each specific file your program 
should correctly determine the length of the extension. If we 
assume a three-character extension is, the directory of the 
drive will look like messy:


             boot B>

             ZX_WIN ZIP

             ******** ZIP

             S_PLAY2d BB

             BACKUM GTR

             BUZZ16_1 MPS

             FL_SH_EI MPS

             KL_F_CUT m!

             SPY MPS

             VIVID ME

             NOSTALGY Y

             s_play2d txt


  Let's try to do: determine what
sequence of three bytes are
valid three-character extensions
and if the file descriptor will be invalid sequence, we assume 
that this single-character file extension.



  First, we list a list of demands
which must satisfy the three-letter extension:
1. All three characters - codes in the range

   # 20 .. # 7F
2. In the extension can not be simultaneously

   large and small Latin letters.
3. Notwithstanding paragraph 2, the expansion of XAS,

   xAS, XaS, xaS are admissible

   (Files with extensions creates

   assembler XAS).
4. If the first character - "B", then the extension

   can not be a three-character, because

   that BASIC-files the next two bytes

   contain the length of the program and variables

   can not be a symbol of expansion.

  Now consider the procedure that defines the length of the 
file extension in accordance with these requirements. 


  The procedure is designed to compile in assembly ZX ASM 3.10, 
and when compiling in another assembler you may need

replace command 'PUSH HL, DE, BC' to three
Team: 'PUSH HL', 'PUSH DE' and 'PUSH BC'.
Accordingly, the command 'POP BC, DE, HL'
need to be replaced by the three teams' POP
BC ',' POP DE 'and' POP HL '.

  Input parameters: HL points to first character of the 
expansion. Register of H should not be zero.


  Output parameters: the registers are
unchanged (except for battery), and the flag
Z carries information about the length of the extension: if the 
flag is set - the three-letter extension, if not set - single 
character. 

  The length of the procedure, only seventy two bytes, it is 
the most optimized, and, apparently, to make it shorter, 
without prejudice to functionality will not work.



    Text of procedure:

ANALYS_EXT PUSH HL, DE, BC

; Check BASIC:


           LD A, (HL)

           CP "B"

           JR Z, ANALYS_NO

; Check XAS, xAS, XaS, xaS:


           RES 5, A

           CP "X"

           JR NZ, ANALYS_EX1


           INC HL

           LD A, (HL)

           RES 5, A

           CP "A"

           JR NZ, ANALYS_EX2


           INC HL

           LD A, (HL)

           CP "S"

           JR Z, ANALYS_YES


           DEC HL
ANALYS_EX2 DEC HL

; Initially, D = # FF, E = 0. Once
; Find symbol A.. Z or a.. Z,
; Add it to D for AND and E on OR.
; In the fifth bit to 0 if the letter is large,
, And 1 if the letter is small. Valid
; Only options 0,0,0 and 1,1,1. If
; Option 0,0,0 (all letters long), then
, 5 th bit D = 0 and 5 th bit E = 0. If
; Option 1,1,1 (all small letters), then
, 5 th bit D = 1 and the 5 th bit E = 1. For
; Other options (when there are 0, 1)
, 5 th bit D = 0 and 5 th bit E = 1. See
; For options 0,0,0 and 1,1,1 5-th bit
, From D XOR E is 0, while for others
; Options will be equal to 1, so
;, We can distinguish between permissible
; Options from unacceptable.

ANALYS_EX1 LD DE, # FF00

           LD BC, 3 * 256 +% 00100000

ANALYS_EX3 LD A, (HL)

           CP C; = CP # 20

           JR C, ANALYS_NO

           CP # 80

           JR NC, ANALYS_NO; not # 20 .. # 7F

           OR C

           CP "a"

           JR C, ANALYS_EX4

           CP "z" +1

           JR NC, ANALYS_EX4

           LD A, (HL)

           AND D

           LD D, A

           LD A, (HL)

           OR E

           LD E, A

ANALYS_EX4 INC HL

           DJNZ ANALYS_EX3


           LD A, D

           XOR E

           AND C

           JR Z, ANALYS_YES

ANALYS_NO OR H; reset Z flag

                   ; (Since H <> 0 then

                   ; Result of the operation

                   ; OR is <> 0)
; When transferring control to ANALYS_YES
; By JR - Z flag will always be installed!

ANALYS_YES POP BC, DE, HL

           RET

  Here, for example, looks like the disk directory where each 
file length extensions determined using this procedure:



           boot B

           ZX_WIN ZIP

           ******** ZIP

           S_PLAY2d B

           BACKUM GTR

           BUZZ16_1 MPS

           FL_SH_EI MPS

           FL_F_CUT m

           SPY MPS

           VIVID M

           NOSTALGY Y

           s_play2d txt


  Now I would like to say a few
words on the drive. In the TR-DOS disk under the name given to 
eight characters, and the name located at offset # F5 in the 
service (Ninth) sector mulevoy track.


  As we see in this sector remain free even three bytes, which 
means that the drive name may be increased to eleven 
characters. Indeed, the 11-character name is often used. But 
there is one nuance ...


  If you want to assign a drive 11simvolnoe name, MSB last
bytes must be set.
The fact that routine printing a string
in TR-DOS (which is used for output
name of the disk when you run CAT)
finishes typing a string or a null byte at a meeting, or when 
the highest bit recently typed characters was

set (printing character MSB
reset). If you typed the common name
eight characters, all goes well, as it is followed by three zero
bytes, but when printing 11-character
name, if the highest bit of the last byte
is not set, then the procedure will not stop printing the name, 
and will print on and further, filling the screen garbage.


  Accordingly, if you write a program
to the disk and want to support
11-character name of the drive, then press
last character name your procedure
print should drop its MSB.



                 Sincerely, Ivan Roshchin.