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Djgpp download windows 8 free. DJGPP Frequently-Asked Questions List |
- The DJGPP Project
For example, 18 times a second there's a timer tick , a hardware interrupt issued by the timer chip that's supposed to advance the system clock. But the handler for the timer tick interrupt is part of BIOS, and it employs real-mode code. So even if a program does nothing to call any real-mode code, some asynchronous system events will do that anyway, and the machine will still crash very promptly. Yes; read on. This software layer is called DOS extender.
With a DOS extender, when a protected-mode program calls a real-mode service, the extender traps the call, switches the CPU to real mode, reissues the call, waits for the service to do its thing, then switches the CPU back into protected mode, and returns to the application code that called the real-mode service.
Hardware interrupts, such as the timer tick and the keyboard interrupt, are also trapped by the extender, and also cause a switch to real mode and back. You might think that these mode switches would considerably slow down the application.
However, in practice, most programs don't call the OS services too often, and even when they do, the peripheral devices accessed by most of these services, such as the hard disk, are so much slower than modern CPUs, that the overhead of the mode switch is hardly ever noticed. It was loaded automatically by every program during its startup. In addition to the usual functions performed by DOS extenders, it also handled some unique DJGPP -related tasks: Loading the application and setting it up for execution.
This is required to overcome deficiencies in stock DOS shells which prevent even the simple task of compiling GCC without extensive hacking of its Makefile s.
FP emulation needs special handling in protected mode. To facilitate graphics programs, go32 allowed to load a driver suitable for the installed video hardware, and worked with the VGA bank switching features to create an illusion of a linear video memory. Using an extender had an important advantage of being able to run on any DOS configuration, since go32 had special code to adapt itself to all known methods of switching into protected mode and managing extended memory.
But it did have a significant drawback as well: the extender was loaded into conventional memory and each instance used about KB of that memory. This was a grave limitation: for example, you couldn't build programs whose Makefile s required more than 2 recursive levels of make invocation because GCC and the compiler passes it invokes require 2 additional levels of program nesting.
DJGPP v2 solves this problem, as described below. It defines several functions that a protected-mode program called a DPMI client can use to perform such tasks as entering protected mode, allocating memory and segment descriptors, calling real-mode services, hooking interrupts, etc.
The DPMI server a. The rest of the functionality, which in v1. The former is a single assembly-language module which is compiled by a special-purpose assembler, called djasm , that is capable of producing bit DOS executables. The second part of the startup is in the library. It consists of several modules written part in C and part in assembly. Here's where the COFF image entry point is, and that is where the stub passes the execution after it loads the program and sets it up.
Here's the short description of what the stub does: Allocate memory for the transfer buffer. This buffer is required for passing data to and from real-mode services.
If DPMI services are not available, the stub loads cwsdpmi. It looks for cwsdpmi. This is required to know how much memory needs to be allocated for the various sections of the DJGPP program. This is done by calling the DPMI functions to allocate segment descriptors and memory for code and data, and set their base address, limit, and privileges.
This entry point is inside the library startup module, described next. Here's what the library startup code does: Make the null page uncommitted. This might sound simple, but is actually quite complicated, due to some peculiarities of DPMI memory allocation. For example, it requires a special bit code that runs in real mode to be loaded into a buffer of conventional memory. This requires to hook some hardware interrupts, e. See Long command lines , and also see Unix-style file-name globbing , for more details.
I already described above how this is done in principle: by calling a special DPMI service provided for that. However, many real-mode services require some data to be passed. For example, when you write the contents of a buffer to a file, the corresponding DOS function requires a pointer to the buffer to be put into the DS:DX pair of registers.
Moreover, the buffer whose pointer is passed to DOS must reside in the first Megabyte of the address space, because real-mode addresses use only 20 bits.
In contrast, protected-mode programs use the full 32 bits for addressing, and all the data is always above the 1MB mark 6. Now, how do we pass such addresses to DOS? This is where the so-called transfer buffer comes to our help. As we saw, this buffer is allocated in conventional memory during the program startup. The buffer is 16KB long by default, but its size can be changed to any value between 2KB and 64KB using the stubedit program.
For example, to write a buffer to a file, the contents of that buffer are copied to the transfer buffer, and the real-mode segment:offset -style address of the transfer buffer is passed to DOS; to read data from a file, the address of the transfer buffer is passed to DOS, and the data put there by DOS is then copied from the transfer buffer to the buffer in protected-mode memory whose address was passed by the calling application.
The startup code stores the real-mode address of the transfer buffer and its size in global variables, which are used by the library function to move data to and from the transfer buffer.
The library also provides special functions to move the data between protected-mode memory and the transfer buffer as fast as possible, and thus to make this overhead smaller. As long as the application calls relatively high-level library functions, such as open , read , write , stat etc. Library functions also provide other specialized processing in some cases. For example, DOS cannot read or write more than 64K bytes in one call, so the library breaks large requests into smaller chunks, each one the size of the transfer buffer, and feeds them to DOS one by one.
As another example, consider memory-allocation functions such as malloc. Instead of allocating blocks off the conventional memory by calling DOS, like real-mode programs do, DJGPP issues DPMI calls to allocate extended memory and provide demand-paged virtual memory, so that all of the available memory and swap space can be used by the application via standard function calls.
Most of these features are built into the C library, but some are provided by the basic development utilities which are part of the DJGPP development environment.
Compatible headers and libraries. For example, DJGPP provides library functions that are usually absent from other DOS- and Windows-based libraries, like popen , glob , statfs , getmntent , getpwnam , select , and ftw. DJGPP makes a point of sticking to Posix or Unix behavior in such cases, even if it means more processing like removing the target file in the case of rename.
A case in point is library functions stat and fstat. Unix programs make extensive use of the inode number and the mode bits returned by these functions. For example, GNU diff examines the inode numbers of the files it is about to compare, and if they are equal, exits immediately on the assumption that both file names point to the same file. Also, the mode bits returned by fstat are usually incorrect. In contrast, the DJGPP implementation of these functions goes out of its way to provide compatible implementations for these functions, and in particular returns meaningful inode numbers 7 , even though it takes quite a lot of code for example, stat code compiled totals about 17KB, together with other library functions it calls.
Such high compatibility makes porting programs very easy. When DOS invokes programs, it limits the length of the command line to characters excluding the program's name. The actual command is stored in the transfer buffer, and a pointer to that buffer is passed to the child program instead of the command line itself.
The startup code of the child program then retrieves the actual command-line arguments and puts them into the argv[] array passed to main. DJGPP also supports the so-called response file method of passing long command lines, whereby the command line is stored on a disk file, and the name of that file is passed as response-file.
For example: ar cq libmylib. All Unix programs assume that any file-name wildcards on their command line were already expanded by the shell, to yield normal file names. But DOS shells don't provide this functionality, so the wildcards would wind up verbatim in the argv[] array. To avoid the need to have special code in every ported program that expands the wildcards, the DJGPP startup code expands the wildcards automatically. DJGPP also provides a special extension: the Thus, the following command would search all files in all the subdirectories, recursively: grep foo Traditionally, the system library function calls the shell to process its argument.
COM is too dumb to be useful in many cases. For example, it doesn't support long command lines, even though DJGPP programs do; it doesn't understand forward slashes in file names; and it doesn't return the exit code of the child program to the parent. COM at all. Instead, it internally emulates its functionality, including redirection and pipes, and invokes the programs directly.
This allows to provide the following important features: Long command lines. See Command line , but here it means that shell commands can have arbitrary length, even though the shell itself doesn't support that!
The emulation code supports the foo ; bar feature of several commands separated by a semi-colon. The emulation of the shell command cd allows Unix-style forward slashes in its argument, and also changes the drive if the argument includes the drive letter. If the environment variable SHELL points to a name like sh or bash , system invokes the shell to do everything, since the internal shell emulation is not sophisticated enough to cover Unix shell functionality.
Shell scripts can be invoked even if the SHELL environment variable doesn't point to a Unix-style shell, provided that the interpreter whose name appears on the first script line after the! COM is only invoked by system to run batch files or commands internal to the shell.
However, system always looks for external programs first, so if you have e. COM has an internal and very much inferior command by that name. Where the original Unix code of make invokes the shell, the DJGPP port simply calls system to execute the commands in rules, and automatically gets support for long command lines and Unix-style shells required to run many Makefile s of Unix origin. The above extended functionality also means that whenever a Unix program calls system , in most cases the same call will work without any changes when compiled with DJGPP.
The result is not only ease of porting, but also less probability to leave subtle bugs in the ported program due to an overlooked fragment which assumes a Unix shell.
This allows to remap some special file names to their DOS equivalents. The implementation of the chroot functionality, which isn't supported directly by DOS and Windows, also uses this file-name conversion. This feature is built into the low-level file-oriented library functions. It allows the application to install a handler for certain filesystem calls, like open , read , fstat , dup , close , etc.
If installed, such a handler is called just before the appropriate primitive is invoked to pass the call to DOS. If the handler returns a non-zero value, it is assumed to have handled the call, and the usual primitive call is bypassed. Otherwise, the library proceeds with calling DOS as usual. This facility provides an easy way of handling special files and devices which DOS and Windows don't support directly.
For example, suppose you need to port code which sends special commands to the terminal device via termcap functions. DOS supports a terminal device, but doesn't support termcap. For example, there are functions to open files, list the files in a directory, create a directory, etc.
For example, it is possible to get and set 3 times for each file, like on Unix, instead of only one time supported by DOS. This run-time detection of the LFN support means that the same executable will run on DOS and on Windows, and will automatically support long file names when it runs on Windows 9X. DOS doesn't support hard and symbolic links. The link library function simulates hard links by copying.
The symlink library function simulates a symbolic link for executable programs only, by creating a 2KB stub which is set up to run the COFF image from the target of the link. Thus, ln -s grep fgrep does what you'd expect. Emacs is special because when it dumps itself during the build process, static and global variables are frozen in the dumped image with the last value they had at the time the program was dumped.
DJGPP has a special facility in the library through which library functions can detect that the program was dumped and restarted. All library functions that need static variables, use this facility to reinitialize them.
These utilities are meant to assist the developer in solving specific tasks common for the DJGPP environment. Some of these utilities are listed below: djtar djtar is a program that unpacks archives but cannot create them. It was originally written to unpack files created by tar , because DOS and Windows lack standard programs for that.
Since the original release, djtar functionality was significantly extended, and now it can unpack. The latter feature is very important when unpacking large distributions, such as emacs-XX. The ability to unzip.
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