COM: Difference between revisions
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(COM files are able to do their own memory organisation on the 8086 - the 64kB limit only applies to code that can't or doesn't make use of segmentation.) |
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COM files are extremely simple executable files. They are useful for loading programs without complications like relocating symbols, reading file headers, etc. However, their simplicity can make their usefulness limited, so you may want to support something more complex but useful, like [[ELF]], once your OS is advanced. |
COM files are extremely simple executable files. They are useful for loading programs without complications like relocating symbols, reading file headers, etc. However, their simplicity can make their usefulness limited, so you may want to support something more complex but useful, like [[ELF]], once your OS is advanced. |
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COM files were used by MS-DOS. They are raw binaries, meaning there is no header data |
COM files were used by MS-DOS. They are raw binaries, meaning there is no header data. |
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MS-DOS creates and stores an info structure starting at offset 0 and ending just before 0x100, called the Program Segment Prefix (PSP). |
MS-DOS creates and stores an info structure starting at offset 0 and ending just before 0x100, called the Program Segment Prefix (PSP). The PSP is made just before starting the COM program. More info about the PSP can be found [http://heim.ifi.uio.no/~stanisls/helppc/program_segment_prefix.html here]. PSP is only required for running DOS programs. |
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COM files usually terminate with a RET instruction. |
COM files usually terminate with a RET instruction. On DOS, the RET pops a empty word from the stack, which results in a interrupt 0x20 (DOS API Exit) being called. |
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===Loading=== |
===Loading=== |
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Revision as of 15:29, 30 October 2019
This is about the executable format, not Microsoft's Component Object Model
| Executable Formats |
|---|
| Microsoft |
| *nix |
| Apple |
COM Files
Introduction
COM files are extremely simple executable files. They are useful for loading programs without complications like relocating symbols, reading file headers, etc. However, their simplicity can make their usefulness limited, so you may want to support something more complex but useful, like ELF, once your OS is advanced.
COM files were used by MS-DOS. They are raw binaries, meaning there is no header data.
MS-DOS creates and stores an info structure starting at offset 0 and ending just before 0x100, called the Program Segment Prefix (PSP). The PSP is made just before starting the COM program. More info about the PSP can be found here. PSP is only required for running DOS programs.
COM files usually terminate with a RET instruction. On DOS, the RET pops a empty word from the stack, which results in a interrupt 0x20 (DOS API Exit) being called.
Loading
To load a COM file you need to load it to a low memory location, point ds and es to the start of the file with an 0x100 org, and jump to the start, with offset 0x100.