NAME    OVERLAY_MANAGER
        ASSUME  CS:CODE,DS:DATA
        
DATA    SEGMENT PUBLIC
DMAPTR  DW      ?                       ;DMA ADDRESS FOR INPUT XFER
PROCNAME DB     8 DUP(?)                ;NAME OF PROCEDURE WHICH WE ARE CALLI
NG
OVLGNUM DB      ?                       ;OVERLAY NUMBER
PROCADR DW      ?                       ;PROCEDURE ADDRESS
MYFCB   DB      36 DUP(?)               ;FCB FOR FILE OPENING
;
;       USRRET STACK CONTAINS RETURN ADDRESS AND
;       SAVED OVERLAY AREA ADDRESS
;       AND OVERLAY GROUP NUMBER        ;(MAX NESTING 25
        DB      129 DUP(?)              ;SAVED RETURN ADDRESSES
USRRET:
OVLAREA DW      ?                       ;LOC OF MOST RECENT OVL AREA
DATA    ENDS

CODE    SEGMENT PUBLIC

M       EQU     Byte Ptr 0[BX]
;---------------------------------------------------------------;
;                                                               ;
;       Overlay Management Module for Pascal/MT+ under PCDOS    ;
;                                                               ;
;       Created:        March 18, 1981                          ;
;       Updated:        February 24, 1983                       ;
;                                                               ;
;---------------------------------------------------------------;
;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++;
;       equates for pertinant information                       ;
;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++;
;
;       THE FOLLOWING THREE EQUATES DESCRIBE DATA PLACED AT THE
;       BEGINNING OF THE .CMD FILE BY LINK/MT+86 AFTER THE
;       STACK POINTER INITIALIZATION CODE.
;       THEY MUST BE REFERENCED WITH A CS OVERRIDE
;
NCBPTR  EQU     000CH
ovlbase EQU     0001h                   ;base prefix for file name
namelen EQU     0000h                   ;length of names (6 or 7 characters)
@XXXX1  EQU     000EH                   ;PTR TO BASE OF OVL_AREA_TAB

TRUE    EQU     -1
FALSE   EQU     0                       ;FOR CONDITIONAL ASSEMBLY
RELOAD  EQU     OFF                     ;NON-RECURSIVE OVERLAY CALLING

;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++;
;       PUBLIC AND EXTRN SYMBOLS                                ;
;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++;

        PUBLIC  @OVL                    ;OVERLAY LOADER
        PUBLIC  @OVS                    ;OVERLAY DISK SET
	extrn	@hlt : near

;###############################################################;
;                                                               ;
;       MAIN ROUTINE - @OVL                                     ;
;       PURPOSE      - LOAD OVERLAY AND CALL PROCEDURE          ;
;---------------------------------------------------------------;
;       ON ENTRY TO @OVL, RETURN ADDRESS POINTS TO OVERLAY CALL ;
;       DATA BLOCK:                                             ;
;                                                               ;
;       +0 : OVERLAY GROUP NUMBER --- 1 BYTE                    ;
;       +1 : OVERLAY PROCEDURE NAME-- 8 BYTES                   ;
;                                                               ;
;###############################################################;
@OVL    PROC    NEAR
        POP     BX
        MOV     AL,CS:M
        MOV     Byte Ptr OVLGNUM,AL
        INC     BX
        MOV     CH,8
        MOV     DX,(Offset PROCNAME)
OVL1:
        MOV     AL,CS:M
        INC     BX
        MOV     SI,DX
        MOV     BYTE PTR [SI],AL
        INC     DX
        DEC     CH
        JNZ     OVL1

        CALL    PSHUSRRET             ;SAVE USER'S RETURN ADDRESSES
        CALL    LOADOVLY               ;GO LOAD IT (IF NECESSARY)
        CALL    FINDPROC               ;GO SEARCH FOR AND FIND PROCNAME
;DOES NOT RETURN IF PROC NOT FOUND
        MOV     BX,(Offset OURRET);PUSH OUR RETURN ADDRESS ON THE STACK
        PUSH    BX
        MOV     BX,Word Ptr PROCADR  ;GET ADDRESS OF PROC WITHIN OVERLAY AREA
        JMP     BX                      ;AND OFF TO USER ROUTINE
OURRET:
        PUSH    AX
        PUSH    BX      ;SAVE POSSIBLE RETURN VALUES
        CALL    POPUSRRET             ;MAY RELOAD OLD OVERLAY GROUP
        POP     BX
        POP     AX      ;GET POSSIBLE RETURN VALUES BACK AGAIN
        JMP     CX                      ;AND BACK TO THE USER (SIMPLE CASE)
@OVL    ENDP

;###############################################################;
;                                                               ;
;       MAIN ROUTINE - @OVS                                     ;
;       PURPOSE      - SET DRIVE NUMBER FOR A SPECIFIC OVERLAY  ;
;       INPUT        - ON STACK : OVLNUM,DRIVE                  ;
;                       OVLNUM : 1..50                          ;
;                       DRIVE   : '@'..'O'                      ;
;                                                               ;
;       PASCAL DEFINITION:                                      ;
;                                                               ;
;       EXTERNAL PROCEDURE @OVS(OVNUM:INTEGER; DRNUM:CHAR);     ;
;                                                               ;
;###############################################################;
@OVS    PROC    NEAR
        POP     BX                      ;RET ADR
        POP     CX                      ;DRIVE NUMBER
        POP     DX                      ;OVERLAY NUMBER
        PUSH    BX
        MOV     AL,CL
        SUB     AL,'@'                  ;MAKE 0..19
	DEC	DX			;ADJUST OVLNUM (COUNT FROM 0)
        MOV     BX,(Offset DRIVETAB)
        ADD     BX,DX
        MOV     CS:M,AL
        RET
@OVS    ENDP

;===============================================================;
;       SUBROUTINE: CALCADDR                                   ;
;       PURPOSE   : CALC OVERLAY AREA ADDRESS BASED ON OVLGNUM  ;
;===============================================================;
CALCADDR PROC   NEAR

        MOV     AL,Byte Ptr OVLGNUM     ;GET REQUESTED GROUP NUMBER
        DEC     AL
        RCR     AL,1
        RCR     AL,1
        RCR     AL,1                    ;DIVIDE BY 8
        AND     AL,1EH
        MOV     DL,AL
        MOV     DH,0
        MOV     BX,CS:Word Ptr @XXXX1      ;GET ADDR OF OVERLAY AREA TABLE
                                        ;POINT TO TABLE ENTRY
        ADD     BX,DX



        MOV     DX,CS:WORD PTR [BX]

        XCHG    BX,DX                   ;BX NOW POINTS TO OVERLAY AREA
        MOV     Word Ptr OVLAREA,BX     ;SAVE IT FOR LATER
        RET
CALCADDR ENDP

;===============================================================;
;       SUBROUTINE: PSHUSRRET                                 ;
;       PURPOSE   : SAVE CONTENTS OF HL, OVERLAY AREA ADDR      ;
;                   AND OVERLAY GROUP NUMBER ON USRRET STACK   ;
;===============================================================;
;        PUBLIC  PSHUSRRET
PSHUSRRET PROC NEAR
        PUSH    BX
        CALL    CALCADDR

        MOV     CX,BX

        POP     DX
        MOV     BX,CS:Word Ptr USRSP      ;GET STACK POINTER
        DEC     BX
        MOV     M,DH                    ;STORE RET ADDR
        DEC     BX
        MOV     M,DL
        DEC     BX
        MOV     M,CH                    ;STORE OVERLAY AREA ADDR
        DEC     BX
        MOV     M,CL
        DEC     BX
        MOV     SI,CX                   ;GET OVERLAY NUMBER
        MOV     AL,CS:BYTE PTR [SI]
        MOV     M,AL
        MOV     CS:Word Ptr USRSP,BX
        RET
PSHUSRRET ENDP

;===============================================================;
;       SUBROUTINE: POPUSRRET                                 ;
;       PURPOSE   : POP RET ADDR, OVERLAY AREA ADDR AND NUMBER  ;
;                   IF RECURSE IS SET TO TRUE THEN THIS ROUTINE ;
;                   WILL CALL LOADOVLY TO RE-LOAD PREVIOUS     ;
;                   OVERLAY IF NECESSARY                        ;
;===============================================================;
;        PUBLIC  POPUSRRET
POPUSRRET PROC NEAR
        MOV     BX,CS:Word Ptr USRSP
        MOV     AL,M
        MOV     Byte Ptr OVLGNUM,AL     ;SAVE OVERLAY GROUP NUMBER
        INC     BX
        MOV     DL,M
        INC     BX
        MOV     DH,M
        INC     BX                      ;DE NOW CONTAINS OVERLAY AREA ADDRESS
        XCHG    BX,DX
        MOV     Word Ptr OVLAREA,BX
        XCHG    BX,DX
        MOV     DL,M
        INC     BX
        MOV     DH,M                    ;DE NOW CONTAINS CALLERS RET ADDRESS
        INC     BX
        MOV     CS:Word Ptr USRSP,BX
        XCHG    BX,DX                   ;PUT REAL ADDR INTO HL
;----------------------------------------------------------------
  IF RELOAD ; THEN RELOAD OLD OVERLAY IF NECESSARY
        PUSH    BX
        MOV     AL,Byte Ptr OVLGNUM
        OR      AL,AL
        JZ      L@1     
        CALL    LOADOVLY               ;ELSE GO LOAD IT IN AGAIN
L@1:
        POP     BX                      ;GET RET ADDR BACK AGAIN
  ENDIF
;----------------------------------------------------------------
        MOV     CX,BX   ;RETURN IT IN CX (1/17/82)
        RET                             ;BACK TO MAIN @OVL ROUTINE
POPUSRRET ENDP

;===============================================================;
;       SUBROUTINE: LOADOVLY                                    ;
;       PURPOSE   : USING OVLADDR AND OVLBASE LOAD THE OVERLAY  ;
;===============================================================;
LOADOVLY PROC   NEAR
        CALL    CALCADDR               ;SETS OVLAREA AND HL-REG
        MOV     AL,Byte Ptr OVLGNUM     ;GET GROUP NUMBER BACK AGAIN
        CMP     AL,CS:M                    ;IS REQUESTED OVERLAY IN THE AREA?
        JNZ     L@2     
        RET                             ;RETURN IF ALREADY LOADED
L@2:
;
;       IF NOT LOADED THEN CONSTRUCT NAME AND LOAD IT
;
;       MOVE    OVLBASE,MYFCB+1,8
        MOV     BX,CS:WORD PTR NCBPTR
        ADD     BX,OVLBASE
        MOV     DX,(Offset MYFCB)+1
        MOV     CH,8
?1:     MOV     AL,CS:M                 ;GET BYTE OF NAME FROM CODE SEG
        MOV     SI,DX
        MOV     BYTE PTR [SI],AL        ;AND STORE IN DATA SEG
        INC     BX
        INC     DX
        DEC     CH
        JNZ     ?1

        MOV     AL,Byte Ptr OVLGNUM
        DEC     AL
        MOV     DL,AL
        MOV     DH,0
        MOV     BX,(Offset DRIVETAB)
        ADD     BX,DX
        MOV     AL,CS:M                 ;GET DRIVE NUMBER FROM TABLE
        MOV     Byte Ptr MYFCB,AL       ;FOR NOW DEFAULT DRIVE ONLY
        MOV     AL,'0'
        MOV     Byte Ptr MYFCB+9,AL
        MOV     AL,Byte Ptr OVLGNUM
        RCR     AL,1
        RCR     AL,1
        RCR     AL,1
        RCR     AL,1
        CALL    CV2HX                   ;CONVERT HIGH NIBBLE
        MOV     Byte Ptr MYFCB+10,AL
        MOV     AL,Byte Ptr OVLGNUM
        CALL    CV2HX                   ;CONVERT LOW NIBBLE
        MOV     Byte Ptr MYFCB+11,AL
        MOV     BX,(Offset MYFCB)+12
        MOV     CH,23                   ;NUMBER OF EXTRA BYTES
LO2:
        MOV     M,0
        INC     BX
        DEC     CH
        JNZ     LO2


        MOV     DX,80H                  ;SET DEFAULT DMA ADDRESS
	call	setdma

        MOV     DX,(Offset MYFCB)
	call	open

        CMP     AL,255
        JNZ     L@3     
        JMP     NOFILE                 ;BR IF FILE NOT FOUND
L@3:
;
;       OK, NOW LOAD IT UNTIL EOF
;
LO3:

        MOV     DX,Word Ptr OVLAREA	;GET OVERLAY AREA ADDRESS
	mov	bx,ds			;save ds
	mov	ax,cs
	mov	ds,ax
	call	setdma
	mov	ds,bx			;RESTORE DS
        MOV     DX,(Offset MYFCB)
	mov	DI,DX
	mov	AX,word ptr [DI+16]	;get file size in bytes
	mov	CL,7			;set up to divide by 128
	shr	AX,CL			;do divide by shifting
	mov	CX,AX			;move number of records into CX
	call	rdran
        RET                            
LOADOVLY ENDP

pcdos	PROC	NEAR	; the call to PCDOS
	push 	es
	int	21h
	pop	es
	ret
pcdos	endp

open:	; file open (#15)
	mov	ah,15
	jmp	pcdos

rdran:	;random read (#39)
	mov	ah,39
	jmp	pcdos

setdma:	;set DMA offset (#26) PCDOS style
	mov	ah,26		; load function number
	jmp	pcdos

;
;       INTERNAL ROUTINE - CV2HX
;       LOW ORDER 4-BITS OF A-REG CONTAIN BINARY NUMBER
;       RETURN ASCII CHAR IN A-REG
;
CV2HX   PROC    NEAR
        AND     AL,0FH
        ADD     AL,'0'
        CMP     AL,03AH
        JNB     L@4     
        RET                             ;RETURN IF A VAILD DIGIT
L@4:
        ADD     AL,7
        RET
CV2HX   ENDP

;===============================================================;
;       SUBROUTINE:     FINDPROC                                ;
;       PURPOSE   :     GIVEN PROCEDURE NAME IN PROCNAME        ;
;                       RETURN ADDRESS OF THIS PROCEDURE IN     ;
;                       PROCADR                                 ;
;===============================================================;
FINDPROC PROC   NEAR
        MOV     BX,Word Ptr OVLAREA     ;GET ADDR OF OVERLAY AREA
        INC     BX
        MOV     DL,CS:M
        INC     BX
        MOV     DH,CS:M                    ;POINT TO TABLE
        XCHG    BX,DX
FP1:
        MOV     AL,CS:M
        OR      AL,AL
        JNZ     L@5     
        JMP     NOPROC
L@5:
        MOV     DX,(Offset PROCNAME)
        MOV     SI,CS:WORD PTR NCBPTR
        MOV     AL,CS:BYTE PTR [SI]
        MOV     CH,AL
        PUSH    BX                      ;SAVE ADDR OF TABLE ENTRY
FP2:
        MOV     SI,DX
        MOV     AL,BYTE PTR [SI]
        CMP     AL,CS:M
        JNZ     FP3                     ;BR IF NO MATCH
        INC     BX
        INC     DX
        DEC     CH
        JNZ     FP2                     ;BR IF MORE TO COMPARE
;
;       WE FOUND IT......
;
        POP     BX                      ;GET BASE ADDR OF TABLE ENTRY
        MOV     DX,8                    ;BYPASS NAME

        ADD     BX,DX
        MOV     DL,CS:M
        INC     BX
        MOV     DH,CS:M
        XCHG    BX,DX
        MOV     Word Ptr PROCADR,BX
        RET                             ;AND EXIT
FP3:
        POP     BX
        MOV     DX,10
        ADD     BX,DX
        JMP     SHORT FP1
FINDPROC ENDP

;===============================================================;
;       ERROR MESSAGE PRINTING ROUTINES                         ;
;===============================================================;
NOFILE:                                ;***OVERLAY FILE NOT FOUND***
;       MOVE    MYFCB+1,NFMSG1+2,8      ;MOVE IN NAME
        MOV     BX,(Offset MYFCB)+1
        MOV     DX,(Offset NFMSG1)+2
        MOV     CX,8
?2:
        MOV     AL,M
        INC     BX
        MOV     SI,DX
        MOV     CS:BYTE PTR [SI],AL
        INC     DX
        DEC     CX
        MOV     AL,CH
        OR      AL,CL
        JNZ     ?2
;       MOVE    MYFCB+9,NFMSG1+11,3     ;MOVE IN EXTENSION
        MOV     BX,(Offset MYFCB)+9
        MOV     DX,(Offset NFMSG1)+11
        MOV     CX,3
?3:
        MOV     AL,M
        INC     BX
        MOV     SI,DX
        MOV     CS:BYTE PTR[SI],AL
        INC     DX
        DEC     CX
        MOV     AL,CH
        OR      AL,CL
        JNZ     ?3
        MOV     AL,Byte Ptr MYFCB
        ADD     AL,'@'
        MOV     CS:Byte Ptr NFMSG1,AL
        MOV     DX,(Offset NFMSG)
        CMP     AL,'@'
        JNZ     WMSG
        MOV     BX,'  '
        MOV     Word Ptr NFMSG1,BX      ;CHANGE "@:" TO "  "
WMSG:
        PUSH    DS
        MOV     AX,CS
        MOV     DS,AX
	mov	ah,9
	call	pcdos
        POP     DS
	call	@hlt

NFMSG:	
        DB      13,10,'Unable to open: '
NFMSG1  DB      40H,':        .   '
        DB      13,10
        DB      '$'
NOPROC:                                ;***PROCEDURE NAME NOT FOUND***
;       MOVE    MYFCB+1,NPMSG2+2,8      ;MOVE IN NAME
        MOV     BX,(Offset MYFCB)+1
        MOV     DX,(Offset NPMSG2)+2
        MOV     CX,8
?4:
        MOV     AL,M
        INC     BX
        MOV     SI,DX
        MOV     CS:BYTE PTR [SI],AL
        INC     DX
        DEC     CX
        MOV     AL,CH
        OR      AL,CL
        JNZ     ?4
;       MOVE    MYFCB+9,NPMSG2+11,3     ;MOVE IN EXTENSION
        MOV     BX,(Offset MYFCB)+9
        MOV     DX,(Offset NPMSG2)+11
        MOV     CX,3
?5:
        MOV     AL,M
        INC     BX
        MOV     SI,DX
        MOV     CS:BYTE PTR [SI],AL
        INC     DX
        DEC     CX
        MOV     AL,CH
        OR      AL,CL
        JNZ     ?5
        MOV     AL,Byte Ptr MYFCB
        ADD     AL,'@'
        MOV     CS:Byte Ptr NPMSG2,AL
        CMP     AL,'@'
        JNZ     NP1
        MOV     BX,'  '
        MOV     CS:Word Ptr NPMSG2,BX
NP1:
;       MOVE    PROCNAME,NPMSG1,8
        MOV     BX,(Offset PROCNAME)
        MOV     DX,(Offset NPMSG1)
        MOV     CX,8
?6:
        MOV     AL,M
        INC     BX
        MOV     SI,DX
        MOV     CS:BYTE PTR [SI],AL
        INC     DX
        DEC     CX
        MOV     AL,CH
        OR      AL,CL
        JNZ     ?6
        MOV     DX,(Offset NPMSG)
        JMP     WMSG

NPMSG   DB      13,10,'Proc: "'
NPMSG1  DB      '        " not found ovl:'
NPMSG2  DB      40H,':        .   '
        DB      13,10
        DB      '$'
;***************************************************************;
;                                                               ;
;       DATA AREA FOR OVERLAY MANAGER                           ;
;                                                               ;
;***************************************************************;
;
;       NOTE THIS TABLE MUST BE IN THE CSEG AREA BECAUSE IS MUST
;       BE INITIALIZED VIA DB AND LINKMT WILL NOT SUPPORT INITIALIZED
;       DATA IN THE DSEG
;
DRIVETAB:                              ;DRIVE TABLES FOR 50 OVERLAYS

        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0
        DB      0

USRSP  DW      (Offset USRRET)        ;USRRET STACK POINTER
;THIS MUST ALSO BE IN CODE SEG
CODE    ENDS
        END