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Diffstat (limited to 'crates/nasm/README.md')
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1 files changed, 44 insertions, 1 deletions
diff --git a/crates/nasm/README.md b/crates/nasm/README.md index 9a44eaa..2d24011 100644 --- a/crates/nasm/README.md +++ b/crates/nasm/README.md @@ -246,7 +246,7 @@ fit in a byte. Hence, you could have a code like follows: If you compile the code with `-D PAL=1`, then the first branch will be taken instead of the second one. -### Unused code +## Unused code This assembler will also issue a warning whenever it finds unreferenced variables or labels. Hence, if you have something like: @@ -269,6 +269,49 @@ an error, as `nasm` can rightly identify that this is dead code. This check can be skipped by providing the `--allow-unused` flag on nasm. +## Cross-mapping references + +This assembler will issue a warning whenever you are referencing an object which +is defined into a segment from another mapping. Some segments, like the +'vectors' one, will reference code that is outside of its mapping. But in some +other configurations, segments cannot make these cross-mapping references so +happily. Imagine that we have an UNROM chip configuration, where "SWAPPABLE" is +a segment that can be swapped according to the specification of this mapper +chip. Then, you could have code like this: + +```asm +.segment "SWAPPABLE" + +.proc foo + rts +.endproc + +.segment "FIXED" + +jsr foo +``` + +Here the assembler will properly detect the address of 'foo' in the context of +the 'SWAPPABLE' segment. But what this assembler doesn't know is that this +segment is swappable. Hence, if the bank being mapped right now is not the one +containing the 'SWAPPABLE' segment, then the address computed for 'foo' and used +in that 'jsr' instruction will point to something else entirely. This would be +similar to a use-after-free bug. + +This is something that can only be inspected at runtime, and so the assembler +cannot be of much help here. Hence, this commit adds a warning so the programmer +can understand the potentially dangerous operation. + +All of that being said, this assembler also adds support for "asan:safe" or +"check:safe", which is a magic comment that the programmer can write to +re-assure the assembler that this operation is fine (e.g. there is a guarantee +that the mapped bank is that one we are expecting). Hence, the code above could +now be written like so: + +```asm +jsr foo ; check:safe +``` + ## Address sanitizer This assembler comes with a set of tools that builds up an "address |