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99 lines
3.6 KiB
Plaintext
99 lines
3.6 KiB
Plaintext
# Cross-compilation
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When Forth words are compiled, they are compiled for the system
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currently running. Those compiled words are tricky to relocate
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because their wordrefs reference offsets within the running
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system.
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If you want to deploy to a new system, you need tricks, and
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those tricks are located at B260, the cross-compilation toolset.
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The mechanism is simple: override ":" so that it adds an offset
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to every wordrefs it compiles.
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What should that offset be? the beginning of the binary being
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built. That offset if the value in the ORG variable, supplied
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by the assembler. It's logical: every binary begins with a bit
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of assembler, which makes every following Forth word aligned
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with this value.
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# Dual-CURRENT
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Although the principle behind cross-compilation is simple, the
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devil's in the details. While building our new binary, we still
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need access to a full-fledged Forth interpreter. To allow this,
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we'll maintain two CURRENT: the regular one and XCURRENT, the
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CURRENT value of the cross-compiled binary.
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XCURRENT's value is a *host* address, not a cross one. For
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example, if our cross binary begins at offset 0x1000 and the
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last word added to it was at offset 0x234, then XCURRENT is
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0x1234.
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During cross compilation, we constantly switch CURRENT (through
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the CURRENT* sysvar, see impl.txt) between the host's and
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XCURRENT.
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As a general rule, switching happens this way: When interpret-
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ing, we're in host mode. When compiling, we're in XCURRENT mode.
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When we encounter an IMMEDIATE during compilation, we execute
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the *host* version of that word. The reason for this is simple:
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any word freshly cross-compiled is utterly un-runable because
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its wordrefs are misaligned under the current host.
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# xcomp unit
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Cross-compilation is achieved through the writing of a cross-
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compilation unit of code, xcomp unit for short.
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The xcomp toolset at B260 alters core words in a deep way, so
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ordering is important. First, we load our tools. Assembler,
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xcomp toolset, etc. The xcomp toolset is designed to not over-
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shadow core words directly, so initial loading, B262, is harm-
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less.
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Now is also the time to define some support words that will not
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be part of our resulting binary, but will be used during xcomp,
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for example, declarations units and macros.
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Then, we load B270 to apply xcomp overrides. From this point on.
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every defining word is messed up and will produce offsetted
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binaries.
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At this point, it's critical to set ORG before spitting any-
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thing. Boot binaries will usually take care of this, so you
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don't have to do it yourself. You just have to make sure that
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you load the boot binary right after loading B270.
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Then, you spit your content following instructions in
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bootstrap.txt.
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After you're done, you can run EMPTY to go back to a usable
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system.
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# Immediate compiling words trickyness
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When using an immediate compiling word such as "IF" during
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xcomp, things are a bit tricky for two reasons:
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1. Immediates used during xcomp are from the host system.
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2. The reference of the word(s) they compile is for the host
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system.
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Therefore, unless the compiled word (for example (?br) compiled
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by IF) has exactly the same address in both the host and guest,
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the resulting binary will be broken.
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For this reason, we re-implement many of those compiling words
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in xcomp overrides, hacking our way through, so that those
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compiling words compile proper guest references. We don't do
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this for all compiling words though. This means that some words
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can't be used in core and drivers, for example, ABORT" and .".
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How to know whether a word can be used?
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1. If it's not an immediate compiling word, it's fine.
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2. If its overriden in B270, it's fine.
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3. Otherwise, you can't cross-compile it.
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