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| -rw-r--r-- | subx.md | 29 |
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diff --git a/subx.md b/subx.md index 4e28a49c..d2b9ad50 100644 --- a/subx.md +++ b/subx.md @@ -87,18 +87,24 @@ the one at address `edx`: ## The syntax of SubX programs -SubX programs map to the same ELF binaries that a conventional Linux system -uses. Linux ELF binaries consist of a series of _segments_. In particular, they -distinguish between code and data. Correspondingly, SubX programs consist of a -series of segments, each starting with a header line: `==` followed by a name -and approximate starting address. +SubX programs consist of functions and global variables. It's very important +the two stay separate; executing data as code is the most common vector for +security issues. Consequently, SubX programs maintain separate code and data +_segments_. To add to a segment, specify it using a `==` header. -All code must lie in a segment called 'code'. +Details of segment header syntax depend on where you want the program to run: + +* On Linux, segment headers consist of `==`, a name and an approximate + starting address (which might perturb slightly during translation) + +* For bootable disks that run without an OS, segment headers consist of `==` + and a name. Boot disks really only have one segment of contiguous memory, + and segment headers merely affect parsing and error-checking. Segments can be added to. ```sh -== code 0x09000000 # first mention requires starting address +== code 0x09000000 # first mention requires starting address on Linux ...A... == data 0x0a000000 @@ -132,12 +138,9 @@ Functions are called using the following syntax: Function arguments must be either literals (integers or strings) or a reg/mem operand using the syntax in the previous section. -A special label is `Entry`, which can be used to specify/override the entry -point of the program. It doesn't have to be unique, and the latest definition -will override earlier ones. - -(The `Entry` label, along with duplicate segment headers, allows programs to -be built up incrementally out of multiple [_layers_](http://akkartik.name/post/wart-layers).) +A special label on Linux is `Entry`, which can be used to specify/override the +entry point of the program. It doesn't have to be unique, and the latest +definition will override earlier ones. Another special pair of labels are the block delimiters `{` and `}`. They can be nested, and jump instructions can take arguments `loop` or `break` that |