IBM AT 5170 Technical Reference page 37

Unlinking Code Example
PUSH
DS
PUSH
ES
CLI Disable interrupts
HOV AX,350FH
DOS get interrupt vector
INT Z1H ES:BX points to first of chain
HOV
CX,ES Pickup segment part of interrupt vector
""
Are we the first handler in the chain?
HOV AX,eS Get code seq into comparable register
CHP
BX,OFFSET ENTRY Interrupt vector in low memory
pointing to your handler's offset?
JNE UNCHAIN_A No, branch
CHP
AX,ex
vector pointing to your
handler's segment?
JNE UNCHAIN_A No, branch
Set interrupt vector in low memory to point to the handler
pointed to
by
your pointer
PUSH DS
MOV DX,WORD PTR CS:FPTR
HOV DS,WORD PTR CS FPTRIZI
MOV AX,2S0PH ; DOS set interrupt vector
INT Z1H
POP DS
JMP UNCHAIN_X
UNCHAIN_A: ex
=
FPTR offset, ES
FPTR segment, ex
=
es
CHP ES:IBX+61,QBQZH Is handler using the appropriate
conventions (is SIGNATURE present in
the interrupt chaining structure)?
JNE exception No, invoke error exception handler
LDS
SI,ES:[BX+2] Get FPTR's segment and offset
CHP SI,OFFSET ENTRY Is this forward pointer pointing to
your handler's offset?
JNE UNCHAIN_B
No, branch
HOV eX,DS
Move to compare
CHP AX,CX Is this forward pointer pointing to
your handler's segment?
JNE
No, branch
Located your handler in the chain
MOV AX,WORD PTR CS:FPTR Get your FPTR's offset
HOV ES: I BX+ZI ,AX Replace offset of FPTR of handler
that points to you
MOV AX,WORD PTR eS:FPTRI2] ; Get your FPTR's segment
MOV ES:[BX+4I,AX Replace segment of FPTR of handler
that points to you
HOV AL,eS:FLAGS
Get your flags
AND AL,FIRST Isolate FIRST flag
OR
ES: IBX
+ 61,AL Set your first flag into prior routine
UNCHAIN_B: MOV BX,SI
Move new offset to BX
PUSH OS
PUSH
ES
JHP UNCHAIN_A Examine next handler in chain
UNCHAIN_X: STI
Enable interrupts
POP
ES
POP DS
System Board
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