Sinclair Club #04 25 мая 2001

Программирование

Programming - Displays a three-character file extensions in the operating system TR-DOS.

 Derivation of three-character file extensions

     in the operating system TR-DOS



     (C) Ivan Roshchin, Moscow



   In the TR-DOS each file is put in
line 16 - byte descriptor
located in the directory of the disk. As can be seen
from a table in the descriptor for
file extension by only one
Symbol:


 Offset Length Commentary
 from the beginning


     0 8 Filename


     August 1 File Extension


     September 2 starting address
                 file (for BASIC -
                 program and
                 variables)


     February 11 Length of the file
                 bytes (for the length of the BASIC program 
without                  taking into account the length of

                 variables)


     January 13 in the length of the file
                 sectors


     January 14 sector number
                 beginning of the file


     January 15 Track Number
                 beginning of the file



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


 Offset Length Commentary
 from the beginning


     0 8 Filename


     August 3 File Extension


     February 11 Length of the file
                 bytes (for BASIC -
                 length of the program without
                 taking into account the length of
                 variables)


     January 13 in the length of the file
                 sectors


     January 14 sector number
                 beginning of the file


     January 15 Track Number
                 beginning of the file



   It is seen that the length of the expansion has increased
by using two bytes, which
previously stored starting address of the file (for
BASIC - the files - the length of the program and
variables).


   If you write a program for
work with files, it would be nice
to include in her three-character output
extensions. But here comes the problem:
not known in advance which files
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 the extension is always considered
three-character, then the directory of the drive will
look 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, then we
assume that this single-character file
extension.


   First, we list a list of demands
that must be satisfied
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

   and can not be symbols

   expansion.


   Now consider the procedure
determining the length of the file extension
According to these requirements.


   The procedure is designed to compile a
Assembler 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 the
first character of the expansion. Register H can not
must be zero.

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

   The length of the procedure, all seventy-two
byte, it is the most optimized, and
seem to make it shorter, without prejudice
for the 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 directory
disk, where each file length
expansion is determined by this
Procedure:

           boot B
           ZX_WIN ZIP
           ******** ZIP
           S_PLAY2d B
           BACKUM GTR
           BUZZ16_1 MPS
           FL_SH_EI MPS
           KL_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 under the name
drive for eight characters, and itself
name is located at offset # F5 in
the service (ninth) sector mulevoy
track.

   As we see in this sector are
free three bytes, and hence the name
disc can be increased to eleven
characters. Indeed, the 11-character name
quite often used. But there is
a caveat ...

   If you want to assign a disk
11 - character name, then MSB
the last byte must be
installed. The fact that the subroutine
print lines in TR - DOS (which
used to display the name of the disc with
command CAT) finishes
print a line or at a meeting of the zero
bytes, or when the MSB of the last
printed character has been established (at
print character MSB cleared).
If you typed the common name of the eight
characters, all goes well, as
followed by three zero bytes, but
printing 11 - character name, if
MSB of the last byte is not
installed, the procedure will not stop
typing a name, and will print on and
continue to fill the screen garbage.

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


  (Radiolyubitel. your PC 7 / 2000)