ZX Club #04 16 сентября 1997

Программирование

Soft group - Intercepting System Errors in Basic and TR-DOS when programming in assmblere (procedure).

    Programmers who try to write
high-quality programs to run on
computers compatible with the ZX Spectrum,
faced with some problems of methodological and practical plan.

    One of them, perhaps the most annoying is the fact that 
when the any read / write drive system

TR-DOS can clear the screen and the message that there was a 
disk error or that the diskette installed security token from 
the record. After that, she asked whether to try the operation 
again, interrupt it or ignore it. For the programmer, a problem 
that appears the need to restore the screen after

Treatment for DOS. In addition, the system changes the number 
of the current flow. In general, a program running in, say, 
windowed interface should be fully processed his mistakes in 
this interface. 

    Next I'll show you how to intercept functions described 
TR-DOS better, in my view method. Unfortunately, in this

If you can not select a certain covering all possible 
situations the algorithm therefore be given a concrete example,

taken from the windowing applied
programs developed by the author. But first a few words about 
how the TR-DOS ROM is drawn to the main computer. 

    As you know, shadow ROM system
TR-DOS is connected to the place of the main ROM
computer when accessing the processor to certain memory 
addresses to read the operation code. One of these addresses is 
the address of # 3D13 - the entry point to serve TR-DOS. This 
connection sets up a special trigger in Beta Disk Interface, 
and additional ROM is connected to the treatment

CPU for the operation code beyond this
Rom. Then the trigger is reset.

    If the TR-DOS why either need to turn to the main memory 
(for example, to clear the screen, display characters, etc.), 
then she does this as follows:



  1) placed on the stack the return address for the current 
part or module TR-DOS; 

  2) placed on the stack address is required routines in the 
main ROM; 

  3) to go at 23,746;

  4) because it is the address of RAM
Shadow ROM is disabled (cleared by hardware trigger);

  5) at this location is situated instruction RET, therefore, 
is taken from the stack address subroutine in the main ROM and 
it runs; 

  6) RET instruction at the end of the running
subroutine will jump back in the shadow ROM system TR-DOS.


    The proposed example shows how
You can apply to the TR-DOS, so that no
allow it to clear the screen and give their
Your Message to the bottom window of the screen. Instead of
Your Message of TR-DOS is given in the window
"Communication systems" used window
interface. In the example, there is recourse to
undescribed here windowing procedures. Projecting their routine
similar purpose, you can insert into place calls to these 
procedures, their specific calls with similar functions.


    In this case, call TR-DOS divided
into 3 phases:


  1) preliminary tasks: setting
interceptors;

  2) immediately call TR-DOS at # 3D13 (with ability to work 
interceptors); 

  3) completing the operation: the removal of interceptors.


    The program is written in assembler mnemonics HiSoft GEN 
Assembler Version 4.1. So the first step - installing 
interceptors. 


  DOSPRE SUB A

         LD (RSTFLG), A; message still DOS
; Not issued

         LD HL, $ $ DOSW

         LD (RSTPTR), HL; pointer in the text, and those 
windows - to enter characters 

         CALL CLKON; show cursor
; An hourglass - a long opera, tion "

         CALL IM1; transfer system
; In interrupt mode 1
, Followed by steps to install the ne; rehvatchikov

  DOSSWP LD HL, DOS_ST; the first area
; Exchange

         LD DE, 23746, the second region

         LD BC, 3; the length of the

  SWAP2 LD A, (DE); exchange content
; Memory areas

         LDI

         DEC HL

         LD (HL), A

         INC HL

         JP PE, SWAP2
, So the team RET at
; 23746 replaced on the team JP DOSEMT

         LD HL, (23633) is a systemic transition; belt CURCHL

         LD C, (HL)

         INC HL

         LD B, (HL); the address formation; botchika display 
the current channel 

         LD DE, (DOS_CH); in DOS_CH address
, Our handler

         LD (DOS_CH), BC; rearrange them

         LD (HL), D

         DEC HL

         LD (HL), E
, So the procedure for deriving the current
; Substituted for a special channel. Set; ka interceptor 
finished. 

         RET

; Directly call DOS
; After a single run DOSPRE, you can
; Many times to run DOS; nuzh after all, but run DOSEND


  DOS PUSH AF

         LD (IY), -1; "no errors"

         SUB A; initial value, the return code of TR-DOS

         LD (23823), A

         LD (23824), A

         POP AF

         CALL # 3D13; run TR-DOS

         LD A, (23823), setting a flag
; CY on return code TR-DOS

         AND A

         RET Z; if code = 0, then
; CY = 0

         SCF

         RET

; Area for temporary storage transition team


  DOS_ST JP DOSEMT

; Temporary storage registers


  DOS_HL DEFW 0

  DOS_BC DEFW 0

; Address of the procedure output to a pipe


  DOS_CH DEFW DOSRST

; Interceptor access to the main ROM;
And if the module is running, hence, TR-DOS
, Calls the subroutine main ROM


  DOSEMT LD (DOS_HL), HL; maintain registration; algebra

         LD (DOS_BC), BC

         POP BC; is the address of the OS; novnom ROM

         PUSH AF

         AND A

         LD HL, # D6B

         SBC HL, BC; address = # D6B?
; (Sub clearing the screen)

         JR Z, DOSCLS; if so, run, eat new module cleanup

         AND A

         LD HL, # 1601, Address = # 1601?
; (Sub-setting the current flow)

         SBC HL, BC

         JR Z, DOSIGN; if so, ignored; fix the call flow does 
not change ; Called routine is not one of

; Intercepted, so it ipolnyaem

         POP AF

         PUSH BC; vozvrashenie address on
; Place

         LD HL, (DOS_HL); return result; Giustra

         LD BC, (DOS_BC)

         RET; execution ordered
; Sub main ROM


  DOSIGN POP AF

         RET

; Non-zero value of this flag indicates
; That the message box TR-DOS is already open, then


  RSTFLG DEFB 0

; Address space of the next character in the
The text box


  RSTPTR DEFW $ $ DOSW

; Routine cleaning windows, replacing
; Routine cleaning of the screen


  DOSCLS POP AF; concrete action, not principal conditions

         LD HL, $ $ DOSW

         LD DE, $ $ DOSW +1

         LD BC, 5 * 1.21

         LD (HL), 255

         LDIR

         LD HL, $ $ DOSW

         LD (RSTPTR), HL

         LD A, (RSTFLG); if the window is not on, indoor

         AND A

         LD A, 28

         CALL Z, OPEN; then open it

         LD A, -1

         LD (RSTFLG), A; change the flag

         JP REDRAW; redraw neighborhood, but

; Subroutine output symbol in the window replacement; sculpt a 
character output in the region "K" ; Screen



  DOSRST PUSH BC

         PUSH DE

         PUSH HL

         LD HL, (RSTPTR)

         LD (HL), A

         INC HL

         LD (RSTPTR), HL

         CALL TEXTON; update the text in the
; Window

         POP HL

         POP DE

         POP BC

         RET

; Cleanup operations after a series of calls
; DOS


  DOSEND LD (IY), -1; "no error occurred"

         CALL IM2; return mode
; Interrupt 2

         CALL CLKOFF; remove cursor
; Hourglass

         CALL DOSSWP; bring back all
; Vector conversion and Indices

         LD A, (RSTFLG); if the window messages; of TR-DOS 
revealed 

         AND A

         CALL NZ, CLOSE; then close it

         RET


    Thus, if the TR-DOS wants to
during operation to give a message Disk
error or Read only, it is the first step
attempt to clean the screen. This attempt will be detected 
DOSEMT and instead of cleaning screen will be executed 
withdrawal message box TR-DOS and its purification.


    Further, TR-DOS tries to open a stream associated with the 
channel "K" for issuing the message. This attempt also

will be caught and ignored.

    After this transition at # 10
system will print a message.
It would seem possible at the same subprogram
DOSEMT "catch" and this situation and do not capture the output 
in the current channel. Experiments have shown that it works, 
but for output numbers TR-DOS uses the procedure

OUT_NUM, which accounts in which case
also intercept and emulate that
awkward and inelegant. It is much easier to intercept the 
output channel at the level of the leader tape channel, as done 
in DOSSWP. 

    As a result of TR-DOS message appears in a beautiful 
multi-color box in the middle of the screen, not the bottom. 
The user can respond to a request in the same way as always. In 
addition, the screen is not broken, that very convenient for 
the programmer. Upon completion of the sequence of calls to 
TR-DOS window closes (in the procedure DOSEND).



      Consult the author

      You may, by writing to:

  257010 Ukraine, Cherkassy-10, PO Box 1529

         Kuritsyna BA