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0113-CTRL-C-HANDLER.md

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0113 Ctrl-C / Int 23h Handler / Program Exit

This CTRL-C handler is also the Exit Program subroutine. When called at SYS:0116, it is assumed that the error code is already set prior in AX. The CTRL-C handler (starting at SYS:0113) simply performs the same exit procedure but with ExitCode = -1 (FFh).

SYS0113: MOV AX,00FF

Set ExitCode to FFh (-1). This only happens when the Program Execution is interrupted by pressing CTRL-C.

SYS0116: XOR CX,CX
SYS0118: XOR BX,BX

SYS:0116 is the entry point for the Program Exit. It first clears the ExitProc Address in BX:CX.

SYS011A: MOV DX,DATA
SYS011D: MOV DS,DX

SYS:011A is also an entry point for stopping the program with an ExitCode in AX.

SYS011F: STI

Enable interrupts.

SYS0120: MOV [ExitCode],AX

Save error code in AX to ExitCode.

SYS0123: MOV AX,CX
SYS0125: OR AX,BX
SYS0127: JZ 0166

Check if there is no ExitProc defined.

SYS0129: MOV AX,[OvrLoadList]
SYS012C: OR AX,AX
SYS012E: JZ 015F

Check if there are no overlays (OvrLoadList) loaded.

SYS0130: MOV ES,AX
SYS0132: ES:
SYS0133: MOV AX,[OvrLoadList]
SYS0136: OR AX,AX
SYS0138: JZ 0155

Get any overlays the current has loaded.

SYS013A: SUB AX,BX
SYS013C: JA 0155
SYS013E: NEG AX
SYS0140: CMP AX,1000
SYS0143: JNB 0155
SYS0145: MOV DX,0010
SYS0148: MUL DX
SYS014A: ADD AX,CX
SYS014C: JB 0155
SYS014E: ES:
SYS014F: CMP AX,[OvrDebugPtr.Segment]
SYS0153: JB 015B

???

SYS0155: ES:
SYS0156: MOV AX,[OvrEmsHandle]
SYS0159: JMP 012C

Load handle of overlays loaded in expanded memory (OvrEmsHandle) into AX then go back to SYS:012C.

SYS015B: MOV CX,AX
SYS015D: MOV BX,ES

Copy address in ES:AX to BX:CX.

SYS015F: SUB BX,[PrefixSeg]
SYS0163: SUB BX,+10

Locate original PSP (usually 10h paragraphs below in memory).

SYS0166: MOV [ErrorAddr.Offset],CX
SYS016A: MOV [ErrorAddr.Segment],BX

Load pointer in BX:CX (PSP Address) to ErrorAddr.

SYS016E: LES BX,[ExitProc]

Get address of current ExitProc into ES:BX.

SYS0172: MOV AX,ES
SYS0174: OR AX,BX
SYS0176: JZ 018B

If ExitProc is undefined then flush buffers (SYS:018B Flush and Close Input and Output Files).

SYS0178: XOR AX,AX
SYS017A: MOV [ExitProc.Offset],AX
SYS017D: MOV [ExitProc.Segment],AX
SYS0180: MOV [InOutRes],AX

Clears the following:

... of the current overlay.

SYS0183: MOV AX,016E
SYS0186: PUSH CS
SYS0187: PUSH AX

Setups up a return address to SYS:016E.

SYS0188: PUSH ES
SYS0189: PUSH BX

Pushes the next ExitProc address in ES:BX and...

SYS018A: RETF

Exit from the next overlay in the chain or back to SYS:016E.

See also: SYS:018B Flush and Close Input and Output Files or go Back