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COM: Difference between revisions
→Introduction: An executable in .com format can manipulate the segment registers and use additional memory once it is running, but the size of the binary itself is limited, and it is loaded as described.
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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.) |
(→Introduction: An executable in .com format can manipulate the segment registers and use additional memory once it is running, but the size of the binary itself is limited, and it is loaded as described.) |
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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 were used by MS-DOS. They are raw binaries, meaning there is no header data. Also, COM files are restricted to the size of one segment (a real-address mode segment, 64kb), minus 256 bytes. When a COM file is loaded, it is assumed that all code and data fits in one segment, and it is loaded to offset 0x100.
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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