Onbekend, 00-00-00
THE USE OF HOOKS
Hooks provide a way to intercept the BASIC interpreter or operating system at
certain points,enabling additional,or alternative,processing to be carried out.To
expand on the last sentence it i s easiest to use an to be carried out.To expand
on the last sentence it iseasiest to use an example.Consider for example the BIOS
call CHPUT(see section 2.3).This routine is used to print text on the screen.Here
is a disassembly of the first few instructions of CHPUT :
PUSH HL SAVE ALL THE REGISTERS USED
PUSH DE
PUSH BC
PUSH AF
CALL FDA4 CALL THE HOOK HCHPU
First of all registers HL,BC,DE and AF are pushed on to the stack.Then memory
address FDA4 is called.Normally,the contents of FDA4 and the following 4
bytes,is a Z80 RET instruction,immediately passing control to the instruction
following the CALL.However,FDA4 is in RAM,so its contents can be altered.Let
the 3 bytes,starting at FDA4 be changed,so that control is passed to another
subroutine at D000,i.e.
FDA4 C3 00 D0 JP D000
Also let the code at D000 be as shown below :
POP HL GET RID OF THE FIRST ITEM ON THE
STACK(i.e.RETURN TO HOOK ADDRESS)
POP AF RESTORE THE REGISTERS
POP BC
POP DE
POP HL
RET EXIT BIOS CALL CHPUT.
Now the item on the top of the stack is 3 more than the address from which
FDA4 was called,i.e.the return address to CHPUT.Therefore,if this is removed
from the stack,this will restore the stack to the state it was in immediately
before calling FDA4.Also,if the next four items are popped as shown above,the
stack will be as it was on entering CHPUT,so the RET instruction will exit the
subroutine CHPUT.Consequently the character which was to be printed will not be !
This example demonstrates how the operating system can be intercepted.The five
bytes,starting at address FDA4,are know as a hook.Similar "hook" exist at many
other points in the operating system and BASIC interpreter,and a list of them,in
alphabetical order,is contained in the following pages.
-----------------------------------------------------------------
+ No RET on hook
+/c No RET on hook if network is present
+/sc Hook doesn't change for sc 8
-----------------------------------------------------------------
+/c FD9A+ HKEYI 0C4A Interrupt handler
FD9F+ HTIMI 0C53 Interrupt handler
FDA4+/scHCHPU 08C0 - CHPUT(A-character;SAVE ALL)
FDA9+/scHDSPC 09E6 Display cursor (no param.)
FDAE+/scHERAC 0A33 Errase cursor (no param.)
FDB3+/scHDSPF 0B2B DSPFNK std routine(no param.)
FDB8+/scHERAF 0B15 ERAFNK std routine(no param.)
FDBD HTOTE 0842 TOTEXT std routine
FDC2+ HCHGE 10CE CHGET std routine
FDC7+ HINIP 071E Copy character set to VDP
FDCC HKEYC 1025 Keyboard decoder
FDD1 HKYEA 0F10 Keyboard decoder
FDD6 HNMI 1398 NMI std routine
FDDB HPINL 23BF PINLIN std routine
FDE0 HQINL 23CC QINLIN std routine
FDE5 HINLI 23D5 INLIN std routine
FDEA HONGO 7810 "ON DEVICE GOSUB"
FDEF+ HDSKO 7C16 "DSKO$"
FDF4 HSETS 7C1B "SET"
FDF9+ HNAME 7C20 "NAME"
FDFE+ HKILL 7C25 "KILL"
FE03 HIPL 7C2A "IPL"
FE08+ HCOPY 7C2F "COPY"
FE0D HCMD 7C34 "CMD"
FE12+ HDSKF 7C39 "DSKF"
FE17+ HDSKI 7C3E "DSKI$"
FE1C HATTR 7C43 "ATTR$"
FE21+ HLSET 7C48 "LSET"
FE26+ HRSET 7C4D "RSET"
FE2B+ HFIEL 7C52 "FIELD"
FE30+ HMKI$ 7C57 "MKI$"
FE35+ HMKS$ 7C5C "MKS$"
FE3A+ HMKD$ 7C61 "MKD$"
FE3F+ HCVI 7C66 "CVI"
FE44+ HCVS 7C6B "CVS"
FE49+ HCVD 7C70 "CVD"
FE4E+ HGETP 6A93 Locate FCB
FE53 HSETF 6AB3 Locate FCB
FE58+ HNOFO 6AF6 "OPEN": not found
FE5D+ HNULO 6B0F "OPEN"
FE62+ HNTFL 6B3B Close I/O buffer 0
FE67 HMERG 6B63 "MERGE/LOAD"
FE6C HSAVE 6BA6 "SAVE"
FE71+ HBINS 6BCE "SAVE"
FE76+ HBINL 6BD4 "MERGE/LOAD"
FE7B+ HFILE 6C2F "FILES"
FE80+ HDGET 6C3B "GET/PUT"
FE85+ HFILO 6C51 Sequential output
FE8A+ HINDS 6C79 Sequential input
FE8F HRSLF 6CD8 "INPUT$"
FE94 HSAVD 6D03"LOC",6D14"LOF",6D25"EOF",6D39"FPOS"
FE99+ HLOC 6D0F "LOC"
FE9E+ HLOF 6D20 "LOF"
FEA3+ HEOF 6D33 "EOF"
FEA8 HFPOS 6D43 "FPOS"
FEAD+ HBAKU 6E36 "LINE INPUT#"
FEB2+ HPARD 6F15 Parse device name
FEB7+ HNODE 6F33 Parse device name
FEBC+ HPOSD 6F37 Parse device name
FEC1 HDEVN This hook not used
FEC6 HGEND 6F8F I/O function despatcher
FECB HRUNC 629A Run-clear
FED0 HCLEA 62A1 Run-clear
FED5 HLOPD 62AF Run-clear
FEDA HSTKE 62F0 Reset stack
FEDF HISFL 145F ISFLIO std routine
FEE4 HOUTD 1B46 OUTDO std routine
FEE9 HCRDO 7328 CR,LF to OUTDO
FEEE HDSKC 7374 Mainloop line input
FEF3 HDOGR 593C Line draw
FEF8 HPRGE 4039 Program end
FEFD+ HERRP 40DC Error handler
FF02 HERRF 40FD Error handler
FF07 HREAD 4128 Mainloop "OK"
FF0C HMAIN 4134 Mainloop
FF11 HDIRD 41A8 Mainloop direct statement
FF16 HFINI 4237 Mainloop finished
FF1B HFINE 4247 Mainloop finished
FF20 HCRUN 42B9 Tokenise
FF25 HCRUS 4353 Tokenise
FF2A HISRE 437C Tokenise
FF2F HNTFN 43A4 Tokenise
FF34 HNOTR 44EB Tokenise
FF39 HSNGF 45D1 "FOR"
+/c FF3E+ HNEWS 4601 Runloop new statement
FF43 HGONE 4646 Runloop execute
FF48 HCNRG 4666 CHRGTR std routine
FF4D HRETU 4821 "RETURN"
FF52 HPRTF 4A5E "PRINT"
FF57 HCOMP 4A94 "PRINT"
FF5C HFINP 4AFF "PRINT"
FF61 HTRMN 4B4D "READ/INPUT" error
FF66 HFRME 4C6D Expression Evaluator
FF6B HNTPL 4CA6 Expression Evaluator
FF70 HEVAL 4DD9 Factor Evaluator
FF75 HOKNO 4F2C Factor Evaluator
FF7A HFING 4F3E Factor Evaluator
FF7F HISMI 51C3 Runloop execute
FF84 HWIDT 51CC "WIDTH"
FF89 HLIST 522E "LIST"
FF8E HBUFL 532D Detokenise
FF93 HFRQI 543F Convert to integer
FF98 HSCNE 5514 Line number to pointer
FF9D HFRET 67EE Free descriptor
FFA2 HPTRG 5EA9 Variable search
FFA7+ HPHYD 148A PHYDIO std routine
FFAC+ HFORM 148E FORMAT std routine
FFB1 HERRO 406F Error handler( _|_° )
FFB6 HLPTO 085D LPTOUT std routine
FFBB HLPTS 0884 LPTSTT std routine
FFC0 HSCRE 79CC "SCREEN"
FFC5 HPLAY 73E5 "PLAY" statement
FFCA H????:
FFCF HBGFD before physical operation with disk controller
FFD4 HENFD after physical operation with disk controller
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