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Kartik K. Agaram 2021-08-01 14:47:30 -07:00
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subx.md
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@ -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