TTL AmigaDOS 68000 overlay supervisor ********************************************************** * * * This version of the overlay supervisor is callable * * from C or Assembler; it will not work for MCC Pascal * * or BCPL which use a calling sequence where the called * * routine's address is saved on the stack. A different * * version of the overlay supervisor is available for * * case. * * * * The main overlay algorithm used here was originally * * described by Richard Evans at Cambridge University * * in about 1979. This code implementation was done by * * Tim King of Metacomco. * * * * Two main data structures are used: * * * * 1. The hunk table * * * * This table, giving the base addresses of all * * hunks currently loaded, is manufactured by * * the loader. Its address is planted in HTAB, * * and its first word is the address of hunk 0. * * Note that the addresses it contains are those * * of the link word of each hunk - not the first * * data word. * * * * 2. The overlay table * * * * This table is manufactured by the linkage * * editor, and its address is planted in OVTAB * * by the loader. It consists of two main * * parts, the ordinate table, and the overlay * * data table. The former is a table which is * * used to remember the ordinate number of each * * segment that is currently loaded. If n is the * * maximum level number, then the table contains * * n+1 entries, and the first word is n+2. The * * extra entry is required because a scan * * through the table below is terminated by a * * zero entry. All the entries are initially * * cleared by the linkage editor. The address * * of the overlay table, plus the contents of * * its first word (n+2), gives the address of * * the overlay data table, which is indexed by * * overlay number. Each entry in this table * * contains the following information, not * * necessarily in this order: * * * * 1. A file mark, generated by 'note', giving * * the position of the segment to be * * loaded. * * 2. The overlay level of the segment. * * 3. The overlay ordinate of the segment. * * 4. The initial hunk number of the segment. * * 5. The number of the hunk containing the * * symbol. * * 6. The offset of the symbol in the hunk. * * * * If an error is detected during overlaying, there is a * * limited choice of sensible actions, since the overlay * * supervisor can make no assumptions concerning the * * environment it was entered from. The action on an * * error is to call an Alert with the code for invalid * * overlay request. * * * * Note that the value in HTAB is a long word address, * * whilst that in OVTAB is a machine address. Each entry * * in the table is four bytes long. * * * ********************************************************** * Change History: * * CARL 05SEP85 * 1) Changes made (suggested by Paul) to the handling * of seek and loadseg errors. When an error * occurs, the program will retry until it is * successful. * 2) Changed bra.s to just bra in OVENT and OVENT2 * macro definitions. * 3) Added the OVERLAYS equate and the necessary OVENT2 * table entries (conditionally assembled). * 4) Eliminated unnecessary include files. * CARL 06SEP85 * 1) CALL macro modified. * 2) CALL use modified. * DTM 15JUN87 * 1) modified to either define or use static OCVs, or * Overlay Call Vectors, as we now call them. * 2) changed entry name to "_ovlyMgr" (note that the * underscore is added automatically, and case IS * very important * 3) added instruction to make sure that only lower * 16 bits of d0 are used in indexing into table; * the new dynamic OCVs only pass a word in d0, * although it's still multiplied by 8 and > 0. * 4) Basically all of this increase the total number * of OCVs to 8k, and furthermore makes it * possible to put this code into a library section NTRYHUNK,CODE INCLUDE "exec/types.i" INCLUDE "exec/tasks.i" INCLUDE "exec/libraries.i" INCLUDE "exec/alerts.i" INCLUDE "libraries/dos.i" xdef ovlyMgr FUNCDEF MACRO * function _LVO\1 EQU FUNC_CNT FUNC_CNT SET FUNC_CNT-6 ENDM FUNC_CNT SET LIB_NONSTD INCLUDE "exec/exec_lib.i" INCLUDE "libraries/dos_lib.i" * Offsets within the overlay table. SysBase EQU 4 OTMARK EQU 0 File position OTLEVL EQU 12 Level OTORD EQU 16 Ordinate OTIHNK EQU 20 Initial hunk for load OTRHNK EQU 24 Hunk containing symbol OTOFF EQU 28 Offset of symbol LIBWORD EQU 23456 * A macro to call a LIBRARY routine: CALLS MACRO MOVEA.L SysBase.w,A6 JSR _LVO\1(A6) ENDM * Call a library, save and restore A6. CALL MACRO ; CALL , MOVE.L A6,-(SP) MOVE.L \2,A6 JSR _LVO\1(A6) MOVE.L (SP)+,A6 ENDM * Now for the first word of the program. FIRST BRA.W NextModule * This next word serves to identify the overlay * supervisor to 'unloader'. DC.L $ABCD Special value * The loader plants values in the next locations. STREAM DC.L 0 Overlay input stream OVTAB DC.L 0 Overlay table (Machine address) HTAB DC.L 0 Hunk table (BCPL address) GLBVEC DC.L 0 Global vector (Machine address) * * The main code of the program. * CNOP 0,4 DC.L LIBWORD DC.B 7,'Overlay' ovlyMgr OV1 MOVEM.L A2-A7/D2-D7,-(SP) MOVE.L OVTAB(PC),A3 Address of overlay table MOVEA.L A3,A4 AND.L #$0FFFF,D0 Make SURE that upper word is ZERO ADD.L (A3),D0 Add value in first word of table LSL.L #2,D0 ADDA.L D0,A3 Address of overlay entry MOVE.L OTLEVL(A3),D0 Get required level LSL.L #2,D0 ADDA.L D0,A4 Pointer to entry in ordinate table MOVE.L OTORD(A3),D0 Get required ordinate CMP.L (A4),D0 Compare with actual BEQ GOTSEG Branch if no load required * Now clear other entries in the ordinate table MOVE.L D0,(A4)+ OV2 TST.L (A4) Table terminated by zero BEQ.S OV3 CLR.L (A4)+ BRA.S OV2 * OV3 MOVE.L OTIHNK(A3),D0 First hunk number of overlay segment ADD.L HTAB(PC),D0 LSL.L #2,D0 Address of entry in hunk table MOVEA.L D0,A4 MOVE.L -4(A4),D0 Previous hunk LSL.L #2,D0 MOVEA.L D0,A6 CLR.L (A6) Clear link field in case of error * Now free all the unwanted hunks ... OV4 TST.L (A4) Scan is terminated by zero BEQ OV5 MOVE.L (A4)+,D1 Address as argument for 'freevec' FREEVEC TST.L D1 BEQ.S FVC1 Return if V = 0 ASL.L #2,D1 Back to mc address MOVEA.L D1,A1 MOVE.L -(A1),D0 D0 = size MOVE.L D0,D1 Take copy ANDI.L #$FF000003,D1 Top byte and bottom 2 bits.. BNE OVERR .. must be clear to be valid length CALL FreeMem,SysBase.w FVC1 BRA.S OV4 and again * Now save CIS and point the file OV5 LEA.L LIBNAME(PC),A1 MOVEQ #0,D0 CALL OpenLibrary,SysBase.w MOVE.L D0,D7 BEQ.S OVERR ADDA.L #OTMARK,A3 Address of file mark * Now seek to the segment and load it. If we fail (probably because * the user removed the disk, then retry until successful. This has * the effect of continually reprompting the user to insert the disk * until he obeys. OV_RETRY: MOVE.L STREAM(PC),D1 New stream MOVE.L (A3),D2 MOVEQ #OFFSET_BEGINNING,D3 Offset to be from beginning CALL Seek,D7 Call 'point(scb,mark,o)' TST.L D0 Check result BMI.S OV_RETRY Branch if error ; Now call the loader again: MOVE.L HTAB(PC),D2 Hunk table (second parameter) MOVEQ #0,D1 Zero parameter for overlay MOVE.L STREAM(PC),D3 CALL LoadSeg,D7 TST.L D0 Check result BMI.S OV_RETRY Branch if error * Add new list to chain after previous one MOVE.L D1,(A6) Add new chain * Here when segment is in store: A3 holds table entry, A1 and D0 are free GOTSEG MOVE.L OTRHNK(A3),D0 Number of hunk containing symbol ADD.L HTAB(PC),D0 LSL.L #2,D0 Machine address of entry in hunk table MOVEA.L D0,A4 MOVE.L (A4),D0 BCPL address of hunk LSL.L #2,D0 ADD.L OTOFF(A3),D0 Add in offset of symbol * The address to jump to is now in D0. Place it in A1 and jump to it. MOVEA.L D0,A1 MOVEM.L (SP)+,A2-A7/D2-D7 restore regs JMP (A1) Jump to new code * Come here on an error during the overlay OVERR MOVE.L #AN_BadOverlay,D7 CALLS Alert RTS * The code here will cause us to jump to the next segment in the list, * which should be the intended entry point. NextModule LEA.L FIRST(PC),A3 Pointer to start of module MOVE.L -4(A3),D7 Next BCPL word ptr ASL.L #2,D7 MC ptr to next module MOVE.L D7,A3 Into address register JMP 4(A3) Tally ho! * Data Area LIBNAME DC.B 'dos.library',0 CNOP 0,4 END