Object Files And Linking ~~~~~~~~~~~~~~~~~~~~~~~~ Devpacs assembler, Genim2, comes with a ' built in ' linker. This means it is possible to assemble a source file straight into an executable file. As this is by far easier than linking an object module using Blink or Alink this is what most people tend to do. The art of linking is never investigated. Object Files ~~~~~~~~~~~~ An object file is a source file that has been assembled into a stage of limbo bordering on executable. What? Well the source has been assembled into hex format, but all label references are left in lookup tables. The reason for doing this is that a particular label may not have been in the source file assembled, but in a source file assembled at an earlier date. When the two source files are linked together using Blink, the lookup tables are cross referenced and all labels are satisfied. It is possible to instruct GenIm that a label referenced in a source file is not defined in that file, but is defined in another file you intend to link with it. This is accomplished using the XREF directive. It is also possible to tell GenIm that a label defined in a source file is going to be referenced in another object file. This is accomplished using the XDEF directive. All other labels used in a module are private to that module, so repetition in another module will not cause duplicate label errors. This process of defining labels in one module for use in another is known as Importing/Exporting. Why Bother? ~~~~~~~~~~~ The main advantage of this method is integrating assembler routines into compiler generated programs. Compilers such as Lattice C, NorthC, PCQ Pascal, DICE and Draco all support this approach. For the assembler programmer, the main benefit is that a module can be written, tested and debugged in one hit. The final source being assembled as an object file ready for linking with future programs. There is no need to ever assemble the code again. An example is needed. Let's try something simple first, an object module that will add two words together: XREF _Answer Where to store result XDEF _Num1,_Num2 Where inputs are found _AddNums moveq.l #0,d0 clear move.l _Num1,d0 add.l _Num2,d0 move.l d0,_Answer _Num1 dc.l 0 _Num2 dc.l 0 Notice how _Answer is defined as being declared in another module, _Answer is said to be Imported into this source. _Num1 and _Num2 are defined as being made available to other modules and are said to be Exported. So how would we use this source. Well first we must assemble it as a Linkable file to disc. Assume we called it 'AddLong.o'. Now a program such as the following could use this routine: XREF _Num1,_Num2 XDEF _Answer Start move.l #5,_Num1 move.l #7,_Num2 jsr _AddNums call routine in other object module move.l _Answer,d0 rts _Answer dc.l 0 Assume this has been assembled as a Linkable file called ' Test.o '. Now the two files would be joined together using Blink from the CLI as follows: Blink FROM Test.o AddLong.o TO ram:Prog The final program generated by linking Test.o and AddLong.o together will be created in ram: and be called Prog. Use Monam2 to load and single step this just to convince yourself it works. Note: It is important to get the order of the files to be linked correct. If the AddLong.o routine was placed first, then execution of the final executable would start in the code generated by AddLong.o, not Test.o as it should do. A Practicle Example ~~~~~~~~~~~~~~~~~~~ One of the many object files I keep handy is a NoiseTracker replayer. This has had a PowerPacker loader added to it to make loading any file from disc a simple affair. The module Imports _GfxBase and _PPBase from any module linked with it. For this reason all programs linked with this file must open the graphics library and powerpacker library. There are two labels Exported, these are PlayFile and StopPlaying. PlayFile requires the address of a NULL terminated filename in register a0, this file is then loaded and played. If a tune is already playing it is stopped first. StopPlaying does not require any parameters, it stops any tunes from playing. Here is an example source that utalises this object module: Incdir sys:include/ Include exec/exec_lib.i XDEF _GfxBase,_PPBase Export to Play.o XREF PlayFile,StopPlaying Import from Play.o Start move.b #0,-1(a0,d0) NULL CLI parameter move.l a0,a5 save filename ptr lea gfxname,a1 a1->lib name moveq.l #0,d0 any version CALLEXEC OpenLibrary open move.l d0,_GfxBase save base pointer beq ERROR lea ppname,a1 a1->lib name moveq.l #0,d0 any version CALLEXEC OpenLibrary open move.l d0,_PPBase save base pointer beq ERROR1 move.l a5,a0 a0->filename jsr PlayFile play the music WAIT btst #6,$bfe001 test LMB bne.s WAIT loop 'till pressed jsr StopPlaying stop music move.l _PPBase,a1 a1->lib base CALLEXEC CloseLibrary close it ERROR1 move.l _GfxBase,a1 a1->lib base CALLEXEC CloseLibrary close it ERROR moveq.l #0,d0 No DOS error rts and finito gfxname dc.b 'graphics.library',0 even _GfxBase dc.l 0 ppname dc.b 'powerpacker.library',0 even _PPBase dc.l 0 As you can see, this is a simple program. With a little work it could be changed into a useable utility, the Play.o file uses the CIA interrupts and is not distorted by disc access. I leave this up to you. To check this example source out, load the file Test.s from the Tutorials directory into GenAm. Assemble as a linkable file to ram: with the name ' test.o '. Now invoke blink with the following: df1:blink FROM ram:test.o df1:tutorials/play.o TO ram:Player When blink has finished, type the following at the CLI: ram:Player df1:Modules/mod.music What easier way of adding a replayer to a program. By the way, this is the replayer used by the latest acc menu system! What Next ~~~~~~~~~ You can now attempt Paul Overaa's Interrupt tutorial in the December issue of Amiga User International. Mark. Okay, I could not resist it. I have made very minor alterations to my ever faithful Int_Start.s routine and converted it into a better replayer. It now starts a tune playing, opens an Intuition window and keeps playing the tune until the window is closed. To create this file, you will need to follow these steps: 1/ Load superplayer.s from the Tutorials of this disc into Genam2. 2/ assemblr to ram: as a Linkable file called player.o 3/ Invoke Blink as follows: Blink FROM ram:player df1:tutorials/play.o TO ram:Player 4/ To test the replayer, enter the following: run Player df1:modules/mod.music 5/ While the music is playing, do something else. Close the small window to stop the music. Hope this has convinced you that object modules have their uses. One last use is doing away with Incbin files. Use a utility I wrote ages ago to convert raw data files into object files. You can even specify the label name to be Exported from the raw data!!!