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-=-=-=-=-=-=-
- *****************************************************************
- *
- MAXFILES Command Handler *
- *
- ----------------------------------------------------------------*
- *
- The MAXFILES command is used to define the maximum number *
- of files that can be opened at one time (1 - 16). A default *
- value of 3 is used for cold starts (i.e. when the disk is *
- booted) or when the FP or INT commands are issued. This value *
- can be changed by altering the contents of MAXDFLT ($AAB1). *
- A diagram representing the structure of the DOS buffers *
- is given at the end of this listing. Note that 595 bytes are *
- reserved for each file and that a DOS buffer is composed of a *
- number of smaller buffers: data sector buffer, track/sector *
- list buffer, file name buffer and chain pointers buffer. The *
- first six bytes of the chain pointers buffer point to the *
- different sections of a given file buffer. The last two bytes *
- in the chain pointers buffer provide a link to the name buffer *
- of the next active file. If there are no more buffers in the *
- DOS buffer chain, the last two bytes of the last chain pointer *
- buffer are zeroed out. Similarly, if A DOS buffer is not *
- being used, the first byte in its filename buffer is $00. *
- Note that the addr of the 1rst DOS buffer at its filename *
- field (ADOSFNB1) is a constant because DOS buffers are built *
- from higher memory downward. (I.e. DOS buffer 1 is built 1rst *
- and occupies the highest memory location whereas DOS buffer *
- 16 is built last & occupies the lowest memory location.) The *
- highest numbered DOS buffer (i.e. the lowest in memory) is *
- always assigned first. *
- MAXFILES cannot normally be used successfully from an exec*
- file. Although the command is indeed executed, the computer *
- is inflicted with a terminal case of amnesia. Disorientation *
- occurs because MAXFILES shuts off the exec flag & CLOSEALL *
- subsequently releases the exec buffer. However, both *
- debilitating events can be prevented simply by NOPing out the *
- STA EXECFLAG instruction at $A253. The MAXFILES command *
- destroys Integer programs and messes up Applesoft strings. *
- This occurs because MEMSIZ & FRETOP (Applesoft) or HIMEM & *
- INTPGMST (Integer) are reset. Under special circumstances, *
- you can prevent some of these problems by placing a premature *
- "RTS" instruction in the ZLNK2NXT ($A82D) routine. You can *
- also issue a MAXFILES command from a running Applesoft program *
- if it is executed before any strings are defined or other files*
- are opened. *
- Programmers are often hard pressed to find a safe place *
- where basic won't write over their custom machine language *
- routines. Short routines are commonly put in page 3 or in the *
- tail end of the input buffer if little input is expected. A *
- few programmers even take great pains to write relocatable *
- routines that can be moved around at will or hidden in an *
- Applesoft program. However, the area between DOS & its buffers*
- represents a sanctuary for machine language programs. Although*
- this region is normally only seven bytes long, it can be *
- enlarged by fooling DOS into building its buffers at a lower *
- location. The procedure is as follows: *
- 1) load ADOSFNBF1 with a value equal to: *
- ADOSFNB1 - (L$ + $26) (where ADOSFNB1 = $9D00, *
- and L$ = hex length of program to be BLOADed. *
- $26 (dec 38) represents the bytes that normally *
- occur between the start of a DOS buffer & the 1rst *
- byte of its filename field or, in decimal notation: *
- POKE 40192, 40192 - (L# + #38). *
- 2) BLOAD your program at: A$ = ADOSFNB1 - L$, or in *
- decimal notation: A = 40192 - L#. *
- 3) call the cold start routine ($3D2), MAXFILES, FP *
- or INT to rebuild the DOS buffers. *
- Greeting programs designed to accommodate Applesoft *
- editing programs often use this technique to install the *
- editor program between DOS and its buffers. (If you are *
- writing commercial (nonturnkey) assembly language programs *
- that are designed to run on different-sized machines or *
- accommadate an altered version of DOS, note the last two *
- paragraphs at the end of this disassembly. These describe how *
- to find moved DOS buffers from an assembly language program *
- that only accesses DOS via indirect addressing.) *
- Inspection of the raw data in a DOS buffer can often *
- reveal protection techniques embedded in a program or its file *
- description (ex. non-listable programs, programs with file *
- names containing back spaces or other illegal control *
- characters, driver programs made "user unloadable" through the *
- use of illegal T/S list sector values, etc.). (See the *
- preamble given with the disassembly of the catalog command for *
- an alternate method of inspecting file descriptions. *
- *
- *****************************************************************
- Execution pattern:
- - Make sure that the EXEC mode is turned off.
- - Close all files.
- - Save the MAXFILES argument in the main variables table
- (MXFILVAL, $A7D4).
- - Build the indicated number of DOS buffers.
- - Check which basic is active and reset the Applesoft
- (MEMSIZ, $73-$74 & FRETOP, $6F-$70) or Integer (HIMEM,
- $4C-$4B & INTPGMST, $CA-$CB) zero-page pointers.
- On entry:
- - A5L ($44) contains the numeric argument that was issued
- with the MAXFILES command. The argument was previously
- screened to insure that it was between 1 and 16.
- Screening was done via a section ($A0AA - $A0C7) in the
- command parsing and processing routines.
(A251)
CMDMXFIL LDA #0 ;Shut off the exec flag.
STA EXECFLAG
LDA A5L ;Get argument issued with MAXFILES
(A258) PHA ;cmd and save it on the stk.
(A259) JSR CLOSEALL
* Close all files.
(A316)
CLOSEALL JSR GETNBF1
* Put address of the 1rst DOS name buffer
* (chain) in the A3L/H pointer.
(A792)
GETFNBF1 LDA ADOSFNB1 ;First link to chain of DOS buffers
LDA ADOSFNB1+1 ;(ie. pt to 1rst filename buffer in chain).
(A798) BNE SETNXPTR ;ALWAYS.
(A7A4)
SETNXPTR STX A3L+1 ;Put addr of 1rst filename buffer in ptr
STA A3L ;(ie. highest name buffer in chain).
TXA ;Get hi-byte of addr back in (a).
GETNXRTN RTS
(A7A9)
(A319) BNE CKIFEXEC ;ALWAYS.
(A31B)
CHKNXBUF JSR GETNXBUF
* Get addr of next filename buffer in chain
* from chain pointers buffer offset 37 & 36
* bytes from 1rst char of present filename
* buffer.
(A79A)
GETNXBUF LDY #37 ;Point the pointer at chain buffer.
LDA (A3L),Y
BEQ GETNXRTN ;If hi byte is 0, the link zeroed out.
TAX ;Save hi byte in (x).
DEY ;Pick up low byte.
LDA (A3L),Y
SETNXPTR STX A3L+1 ;Stick addr of filename buffer in ptr.
STA A3L
TXA ;Condition z-flag in relation to hi byte.
GETNXRTN RTS
(A7A9)
(A31E) BEQ CLOSERTS ;Link zeroed out, so now all files closed.
(A320) ;Exit CLOSEALL via this route.
CKIFEXEC JSR CKEXCBUF
* Check if current filename buffer
* belongs to an exec file.
(A7AF)
CKEXCBUF LDA EXECFLAG ;Check to see if EXECing.
BEQ NOTEXCBF ;No sweat - not running exec file.
LDA EXECBUFF ;We are EXECing, there4 chk if addr of
CMP A3L ;current filename buffer same as that
BNE CKEXCRTN ;for buffer belonging to EXEC.
LDA EXECBUFF+1 ;Maybe - low bytes matched, now chk hi bytes.
CMP A3L+1
BEQ CKEXCRTN ;Exec buffer = current buffer.
NOTEXCBF DEX ;Not EXECing, there4 reduce (x) to make sure z-flag off.
(A7C2) ;(P.S. (x) was orig conditioned to a large non-zero
;value on entry to GETFNBF1. There4, if now DEX then
(A7C3) ;can be sure z-flag will be off.)
CKEXCRTN RTS ;If execing, return with z-flag on.
(A323) BEQ CHKNXBUF ;EXEC WAS ACTIVE SO DON'T CLOSE ITS BUFFER
;OUT OR WILL END UP IN NEVER-NEVER-LAND.
;After all, don't want to close buffer
;if we are using it to exec (ie. would
;be like burying ourselves alive)!!!
(A325) JSR GETFNBY1 ;Get the first byte in the DOS name buffer.
* Get first byte from DOS name buffer.
(A7AA)
GETFNBY1 LDY #0 ;Buffer is free if 1rst byte = $00.
LDA (A3L),Y ;If buf occuppied, the 1rst byte = 1rst
(A7AE) RTS ;char of filename which owns buffer.
(A328) BEQ CHKNXBUF ;This one is already closed, so go back
;to close rest of files.
(A32A) JSR CLOSEONE ;Close the open file.
(A2FC)
CLOSEONE JSR CKEXCBUF
* Check if current filename buffer
* belongs to an exec file.
(A7AF)
CKEXCBUF LDA EXECFLAG ;Check to see if EXECing.
BEQ NOTEXCBF ;No sweat - not running exec file.
LDA EXECBUFF ;We are EXECing, there4 chk if addr of
CMP A3L ;current filename buffer same as that
BNE CKEXCRTN ;for buffer belonging to EXEC.
LDA EXECBUFF+1 ;Maybe - low bytes matched,
CMP A3L+1 ;so now check hi bytes.
BEQ CKEXCRTN ;Exec buffer = current buffer.
NOTEXCBF DEX ;Not EXECing, so DEX to make sure z-flag
(A7C2) ;is off. (P.S. (x) was orig conditioned
;to a large non-zero val on entry to
;GETFNBF1. There4, if now DEX then
(A7C3) ;can be sure z-flag will be off.)
CKEXCRTN RTS ;If execing, return with z-flag on.
(A2FF) BNE FREEBUFF ;ALWAYS BRANCH IF CLOSEONE IS
;ACCESSED VIA CLOSEALL.
(A301) LDA #0 ;Shut the exec flag off.
(A303) STA EXECFLAG ;NOTE: THIS INSTRUCTION IS NEVER CARRIED
;OUT IF ACCESSED VIA CLOSEALL. (An active exec file
;was already detected and skipped by the "BEQ CHKNXBUF"
(A306) ;instruction at $A323.)
FREEBUFF LDY #0
TYA ;Free up the DOS buffer by poking a $00 in the
STA (A3L),Y ;first byte of the filename field.
(A30B) JSR BUFS2PRM
* Get addresses of the DOS buffers from chain
* buffer and put then in the FM parameter list.
(A74E)
BUFS2PRM LDY #30 ;Get addresses of FM Work buffer,
ADRINPRM LDA (A3L),Y ;T/S list buffer, Data sec buffer and
STA WRKBUFFM-30,Y ;NEXT name buf from the chain pointer
INY ;buffer & put them in the FM parameter list.
CPY #38 ;(P.S. Addr of NEXT filename buffer is
BNE ADRINPRM ;NOT used by DOS.)
(A75A) RTS
(A30E) LDA #2 ;Stick opcode for CLOSE function
STA OPCODEFM ;in the FM parameter list.
(A313) JMP FMDRIVER ;Get ready to do the function.
------------
(A6A8)
FMDRIVER JSR FILEMGR ;Call the file manager
;to execute the close function.
(AB06)
FILEMGR TSX ;Save stk pointer so we can rtn to caller.
STX STKSAV ;Save stk pointer.
(AB0A) JSR RSTRFMWA ;Copy contents of FM work buf (in DOS chain)
;to FM work area (not in buffer chain).
(AE6A)
RSTRFMWA JSR SELWKBUF ;Find FM work buf.
* Get the addr of
* the FM work buffer
* from the FM parm
* list & stick it
* it in the A4L/H
* pointer.
(AF08)
SELWKBUF LDA #0
(AF0A) BEQ PT2FMBUF
(AF12)
PT2FMBUF LDA WRKBUFFM,X
STA A4L
LDA WRKBUFFM+1,X
STA A4L+1
(AF1C) RTS
(AE6D) LDY #0 ;Zero out rtn code
(AE6F) STY RTNCODFM ;in FM parm list
(AE72) ;to signal no errs.
STORFMWK LDA (A4L),Y ;Copy FM work buffer
STA FMWKAREA,Y ;(in chain) to FM
INY ;wrk area (nonchain).
CPY #45
BNE STORFMWK
CLC ;Why?????
(AE7D) RTS
(AB0D) LDA OPCODEFM ;Check if opcode is legal.
CMP #13 ;(Must be less than 13.)
BCS TOERROP ;Opcode too large, got range error.
ASL ;Double val of opcode & get addr of
TAX ;appropriate function handler from tbl.
LDA FMFUNCTB+1,X ;Stick addr on stack (hi byte first)
PHA ;& then do a "stack jump" to the appropriate
LDA FMFUNCTB,X ;function handler.
PHA
(AB1E) RTS
.
.
(AC06) .
FNCLOSE .
.
.
(See dis'mbly of CLOSE function.)
.
.
- if necessary, free up any sectors that
were previously allocated to the file
but not needed.
- also updates the file size, fixes up
links & updates data, VTOC, T/S list
& directory sectors if necessary.
.
.
(RTS)
============
TOERROP JMP RNGERROP ;To $B363 - see dis'mbly of errors.
(AB1F) ------------
* Return here afterr doing the CLOSE function.
* (Cause after @ function is done, use stack
* to get back to the original caller.)
(A6AB)
AFTRFUNC BCC FMDRVRTN ;(c) = 0 = no errors.
LDA RTNCODFM ;Get error code from FM parameter list.
CMP #5 ;End-of-data error?
(A6B2) BEQ TOAPPTCH ;Yes. Got a zeroed-out T/S link or a
;zeroed-out data pair in T/S list.
;(Not applicable to the close function.)
(A6B4) JMP OTHRERR ;(See dis'mbly of errors.)
------------
(A6C3)
FMDRVRTN RTS ;Return to caller of FMDRIVER.
(A32D) JMP CLOSEALL ;Go to CLOSERTS via CLOSEALL.
------------
CLOSERTS RTS
(A330) ============
(A25C) PLA ;Get argument issued with maxfiles command
STA MXFILVAL ;and save it in the main variable table.
(A260) JMP BILDBUFS ;Go build maxfiles # of buffers.
------------
- Point A3L/H at FIRST DOS FILENAME BUFFER. Example calcs for FIRST set of DOS
- (Note: First DOS name buffer is lowest buffers on a 48K machine
- numbered but highest in memory.) ----------------------------------
(A7D4)
BILDBUFS SEC ;Wasted byte, might as well be a "NOP".
LDA ADOSFNB1 ;Get addr of 1rst filename field
STA A3L ;and put it in A3L/H.
LDA ADOSFNB1+1
(A7DD) STA A3L+1 (A3L/+1) = ($9CD3)
- Get # of maxfiles wanted & store it
- in the temporary counter (TEMPBYT).
(A7DF) LDA MXFILVAL
(A7E2) STA TEMPBYT
- Free buffer by zeroing out the
- 1rst byte of the filename field.
(A7E5)
ZDOSBUFN LDY #0 (y) = 0
TYA (a) = 0
(A7E8) STA (A3L),Y (9CD3),0: 00 --> 9CD3:00
- Point link in chain pointers buffers
- at the FM WORK BUFFER.
(A7EA) LDY #30 ;Set (y) to index 1rst link (y) = 1E
;in chain pointers buffer.
(A7EC) SEC
LDA A3L ;Subtract 45 from low byte of addr (a) = D3
SBC #45 ;of filename buf to calc low byte (a) = D3 - 2D = A6
STA (A3L),Y ;of addr of FM work buf & put it in 9CD3,1E: A6 --> 9CF1: A6
PHA ;the chain pointer & on the stk. (a) = A6
LDA A3L+1 ;Subtract carry from hi byte of addr of (a) = 9C
SBC #0 ;filename buf to get hi byte of FM wrk buf. (a) = 9C - 0 = 9C
(A7F8) INY ;Kick up (y) to index addr of hi byte of (y) = 1F
;link in chain pointers.
(A7F9) STA (A3L),Y ;Store hi byte of addr of FM wrk buf 9CD3,1F: 9C --> 9CF2: 9C
;in the link.
;(Note: Above calculations affect (a)
;but not A3L/H.)
- Point link in chain pointers buffer
- at the T/S LIST SECTOR BUFFER.
(A7FB) TAX ;Put hi byte of addr of FM wrk buf in (x) = 9C
(A7FC) DEX ;(x) & kick it down so it indexes hi byte (x) = 9B
;of T/S list buf. (T/S list buf is $100
;bytes long.)
(A7FD) PLA ;Get low byte of addr of FM wrk buf back (a) = A6
PHA ;from stack.
INY ;Kick index up to link in chain pointers. (y) = 20
(A800) STA (A3L),Y ;Put low byte of addr of FM wrk buf in 9CD3,20: A6 ---> 9CF3: A6
;link buffer's pointer.
(A802) TXA ;Get hi byte of T/S list buf in (a). (a) = 9B
INY ;Kick up index in chain pointers buf. (y) = 21
(A804) STA (A3L),Y ;Set hi byte of link in chain pointers. 9CD3,21: 9B --> 9CF4: 9B
- Point the link in the chain pointers
- buffer at the DATA SECTOR BUFFER.
(A806) TAX ;Put hi byte of addr of T/S list buf (a) = 9B
(A807) DEX ;in (x) & kick it down to correspond to (x) = 9A
;hi byte of addr of data sec buf.
(A808) PLA ;Get low byte of T/S list buf from (a) = A6
(A809) PHA ;stack & use it for low byte of data Saved (a) = A6
;sec buf (cause they are exactly 1 page
;apart).
(A80A) INY ;Kick up index to chain pointers buf. (y) = 22
(A80B) STA (A3L),Y ;Put low byte of addr of data sec buf in 9CD3,22: A6 --> 9CF5: A6
;the link buf.
(A80E) INY ;Kick up index to chain pointers buf. (y) = 23
TXA ;Get hi byte of adr of T/S list buf & (a) = 9A
(A80F) STA (A3L),Y ;designate it as hi byte of link. 9CD3,23: 9A --> 9CF6: 9A
- Reduce counter for number
- of buffers built so far.
(A811) DEC TEMPBYT ;If counter goes to zero, then just
(A814) BEQ ZLNK2NXT ;did last buffer & should zero out link.
- Not done all bufs yet so point
- link in chain pointers at NEXT
- FILENAME BUFFER.
(A816) TAX ;Set (x) = low byte of addr of data (a) = 9A
;sector buffer.
(A817) PLA ;Get low byte of addr of data sec buf (a) = A6
;back off of stack.
(A818) SEC ;Subtract 38 from low byte of data sec
SBC #38 ;to index NEXT filename field buf. (a) = A6-26 = 80
INY ;Kick up index to chain pointers buf. (y) = 24
STA (A3L),Y ;Store low byte of next filename buf 9CD3,24: 80 --> 9CF7: 80
PHA ;in link & then save it on stk. Saved (a) = 80
TXA ;Get hi byte of data sec buf from (x) (a) = 9A
(A820) SBC #0 ;& subtract carry (in case cross (a) = 9A
;page boundary) to get (a) = hi byte
;of NEXT filename field buffer.
(A822) INY ;Kick index up to chain pointers buf (y) = 25
(A823) STA (A3L),Y ;& store hi byte of NEXT filename field 9CD3,25: 9A --> 9CF8: 9A
;in link.
- Set A3L/H to point to NEXT
- filename field buffer.
(A825) STA A3L+1 A3L+1 = 9A
PLA (a) = 80
STA A3L A3L = 80 therefore, (A3L/+1) --> ($9A80)
(A82A) JMP ZDOSBUFN ;Go back to free NEXT DOS filename
------------ ;buffer & build more DOS bufs.
- No more buffers to build so zero out
- the link that would normally point to
- the next filename field buffer.
(A82D)
ZLNK2NXT PHA ;Save low byte of addr of data buf on stk.
LDA #0 ;Zero out link to next filename field buf.
INY
STA (A3L),Y
INY
(A834) STA (A3L),Y
- Check which basic is active.
(A836) LDA ACTBSFLG ;Check if active basic is FP or Integer.
(A839) BEQ SETINTPT ;Branch if Integer.
- Using Applesoft so initialize
- MEMSIZ & FRETOP (string storage)
- to a value 1 byte greater than
- highest memory location available
- to basic program.
(A83B) PLA
STA MEMSIZ+1
STA FRETOP+1
PLA
STA MEMSIZ
STA FRETOP
(A845) RTS ;Exit to caller of maxfiles cmd.
============ ;Normally exits to AFTRCMD ($A17D) in the
;command parsing and processing routines.
- Using Integer, so set HIMEM
- & program pointer (INTPGMST).
(A846)
SETINTPT PLA
STA HIMEM+1
STA INTPGMST+1
PLA
STA HIMEM
STA INTPGMST
(A850) RTS ;Exit to caller of maxfiles cmd.
============ ;Normally exits to AFTRCMD ($A17D) in the
;command parsing and processing routines.
DOS BUFFER DEFAULT LOCATIONS ON A 48K MACHINE
l Memory available to Applesoft program l
l l
Addrs on 48 K system l l Offset from
(when MXFILVAL = 3) l l start of DOS buffer
l================================================================l
>>>>>>>>>>>>>>> $9600 l l $00
^ l l
^ l l
^ l l
^ l l
^ l l
^ l Data Sector Buffer l
^ l (DOS Buffer 3) l
^ l (256 bytes) l
^ l l
^ l l
^ l l
^ l l
^ l l
^ l l
^ l l
^ l l
^ $96FF l l $0FF
^ l----------------------------------------------------------------l
^ >>>>>>>>> $9700 l l $100
^ ^ l l
^ ^ l l
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l T/S List Sector Buffer l ^
^ ^ l (DOS Buffer 3) l Z
^ ^ l (256 bytes) l E
^ ^ l l R
^ ^ l l O
^ ^ l l S
^ ^ l l
^ ^ l l O
^ ^ l l U
^ ^ l l T
^ ^ l l ^
^ ^ $97FF l l $1FF ^
^ ^ l----------------------------------------------------------------l ^
^ ^ >>> $9800 l l $200 ^
^ ^ ^ l l ^
^ ^ ^ l l ^
^ ^ ^ l FM Work Area Buffer l ^
^ ^ ^ l (DOS Buffer 3) l ^
^ ^ ^ l (45 bytes) l ^
^ ^ ^ l l ^
^ ^ ^ l l ^
^ ^ ^ $982C l l $22C ^
^ ^ ^ l----------------------------------------------------------------l ^
^ ^ ^ $982D l l $22D <<<< ^
^ ^ ^ l l ^ ^
^ ^ ^ l Filename Field Buffer l ^ ^
^ ^ ^ l (DOS Buffer 3) l ^ ^
^ ^ ^ l (30 bytes) l ^ ^
^ ^ ^ $984A l l $24A ^ ^
^ ^ ^ l----------------------------------------------------------------l ^ ^
^ ^ ^...$984B l l $24B ^ ^
^ ^.................l Chain Pointers Buffer l ^ ^
^.......................l (DOS Buffer 3) l ^ ^
$9852 l (8 bytes) ($0000) l $252...........^
l=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-l ^
>>>>>>>>>>>>>>> $9853 l l $00 ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l Data Sector Buffer l ^
^ l (DOS Buffer 2) l ^
^ l (256 bytes) l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ $9952 l l $0FF ^
^ l----------------------------------------------------------------l ^
^ >>>>>>>>> $9953 l l $100 ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l T/S List Sector Buffer l ^
^ ^ l (DOS Buffer 2) l ^
^ ^ l (256 bytes) l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ $9A52 l l $1FF ^
^ ^ l----------------------------------------------------------------l ^
^ ^ >>> $9A53 l l $200 ^
^ ^ ^ l l ^
^ ^ ^ l l ^
^ ^ ^ l FM Work Area Buffer l ^
^ ^ ^ l (DOS Buffer 2) l ^
^ ^ ^ l (45 bytes) l ^
^ ^ ^ l l ^
^ ^ ^ l l ^
^ ^ ^ $9A7F l l $22C ^
^ ^ ^ l----------------------------------------------------------------l ^
^ ^ ^ $9A80 l l $22D <<<<<<<<<<<
^ ^ ^ l l ^ ^
^ ^ ^ l Filename Field Buffer l ^ ^
^ ^ ^ l (DOS Buffer 2) l ^ ^
^ ^ ^ l (30 bytes) l ^ ^
^ ^ ^ $9A9C l l $24A ^ ^
^ ^ ^ l----------------------------------------------------------------l ^ ^
^ ^ ^...$9A9D l l $24B ^ ^
^ ^.................l Chain Pointers Buffer l ^ ^
^.......................l (DOS Buffer 2) l ^ ^
$9AA5 l (8 bytes) l $252 ...^ ^
l=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-l ^
>>>>>>>>>>>>>>> $9AA6 l l $00 ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l Data Sector Buffer l ^
^ l (DOS Buffer 1) l ^
^ l (256 bytes) l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ l l ^
^ $9BA5 l l $0FF ^
^ l----------------------------------------------------------------l ^
^ >>>>>>>>> $9BA6 l l $100 ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l T/S List Sector Buffer l ^
^ ^ l (DOS Buffer 1) l ^
^ ^ l (256 bytes) l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ l l ^
^ ^ $9CA5 l l $1FF ^
^ ^ l----------------------------------------------------------------l ^
^ ^ >>> $9CA6 l l $200 ^
^ ^ ^ l l ^
^ ^ ^ l l ^
^ ^ ^ l FM Work Area Buffer l ^
^ ^ ^ l (DOS Buffer 1) l ^
^ ^ ^ l (45 bytes) l ^
^ ^ ^ l l ^
^ ^ ^ l l ^
^ ^ ^ $9CD2 l l $22C ^
^ ^ ^ l----------------------------------------------------------------l ^
^ ^ ^ $9CD3 l l $22D <<<< ^
^ ^ ^ l l ^ ^
^ ^ ^ l Filename Field Buffer l ^ ^
^ ^ ^ l (DOS Buffer 1) l ^ ^
^ ^ ^ l (30 bytes) l ^ ^
^ ^ ^ $9CF0 l l $24A ^ ^
^ ^ ^ l----------------------------------------------------------------l ^ ^
^ ^ ^...$9CF1 l ex. A6, 9C l $24B ^ ^
^ ^.................l ex. A6, 9B Chain Pointers Buffer l ^ ^
^.......................l ex. A6, 9A (DOS Buffer 1) l ^ ^
$9CF8 l (8 bytes) ex. 80, 9A l $252...........^
l=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-l ^
$9CF9 l (7 unused bytes) l $253 ^
$9CFF l l $259 ^
l================================================================l ^
$9D00 l (ADOSFNB1/+1)---> l..........^
l DOS l
l l
(Normally, the byte at $3D2 corresponds to the page
number of the start of DOS. This byte can there4 be used
to determine if DOS is at its "normal" location.
If $3D2 contains a value less than #$9D then the computer
has less than 48K of RAM. A value equal to #$9D means that
DOS is at its normal 48K location whereas a larger value
describes a machine with at least 48K of memory. In the
later instance, DOS may reside on the language card.)
Note that the first two bytes of DOS point to the first
DOS buffer at its filename field. Therefore, once you
find the start of DOS (via the high byte of the warm-
start vector), you can use the first two bytes of DOS to
locate the DOS buffer chain even if an editor program is
installed between DOS and its buffers.