doc: add cross.txt

blk/260

@@ -1,7 +1,7 @@
 Cross compilation program
 
 This programs allows cross compilation of boot binary and
-icore. Run "262 LOAD" right before your cross compilation and
+core. Run "262 LOAD" right before your cross compilation and
 then "270 LOAD" to apply xcomp overrides.
 
 This unit depends on a properly initialized z80a with ORG and

blk/261

@@ -3,7 +3,4 @@ automatically shadowed to allow the harmless inclusion of
 this unit. This shadowing has to take place in your xcomp
 configuration.
 
-See example in /emul/xcomp.fs
-
-Why limit ourselves to icore? Oh, I've tried cross-compiling
-the whole shebang. I tried. And failed. Too dynamic.
+See /doc/cross.txt for details.

doc/bootstrap.txt

@@ -1,7 +1,7 @@
 # Bootstrap guide
 
-You want to deploy Collapse OS on a new system? Read usage.txt
-and impl.txt, then continue here.
+You want to deploy Collapse OS on a new system? Read usage.txt,
+impl.txt, cross.txt, then continue here.
 
 What is Collapse OS? It is a binary placed either in ROM on
 in RAM by a bootloader. That binary, when executed, initializes

doc/cross.txt

@@ -0,0 +1,73 @@
+# Cross-compilation
+
+When Forth words are compiled, they are compiled for the system
+currently running. Those compiled words are tricky to relocate
+because their wordrefs reference offsets within the running
+system.
+
+If you want to deploy to a new system, you need tricks, and
+those tricks are located at B260, the cross-compilation toolset.
+
+The mechanism is simple: override ":" so that it adds an offset
+to every wordrefs it compiles.
+
+What should that offset be? the beginning of the binary being
+built. That offset if the value in the ORG variable, supplied
+by the assembler. It's logical: every binary begins with a bit
+of assembler, which makes every following Forth word aligned
+with this value.
+
+# Dual-CURRENT
+
+Although the principle behind cross-compilation is simple, the
+devil's in the details. While building our new binary, we still
+need access to a full-fledged Forth interpreter. To allow this,
+we'll maintain two CURRENT: the regular one and XCURRENT, the
+CURRENT value of the cross-compiled binary.
+
+XCURRENT's value is a *host* address, not a cross one. For
+example, if our cross binary begins at offset 0x1000 and the
+last word added to it was at offset 0x234, then XCURRENT is
+0x1234.
+
+During cross compilation, we constantly switch CURRENT (through
+the CURRENT* sysvar, see impl.txt) between the host's and
+XCURRENT.
+
+As a general rule, switching happens this way: When interpret-
+ing, we're in host mode. When compiling, we're in XCURRENT mode.
+
+When we encounter an IMMEDIATE during compilation, we execute
+the *host* version of that word. The reason for this is simple:
+any word freshly cross-compiled is utterly un-runable because
+its wordrefs are misaligned under the current host.
+
+# xcomp unit
+
+Cross-compilation is achieved through the writing of a cross-
+compilation unit of code, xcomp unit for short.
+
+The xcomp toolset at B260 alters core words in a deep way, so
+ordering is important. First, we load our tools. Assembler,
+xcomp toolset, etc. The xcomp toolset is designed to not over-
+shadow core words directly, so initial loading, B262, is harm-
+less.
+
+Now is also the time to define some support words that will not
+be part of our resulting binary, but will be used during xcomp,
+for example, declarations units and macros.
+
+Then, we load B270 to apply xcomp overrides. From this point on.
+every defining word is messed up and will produce offsetted
+binaries.
+
+At this point, it's critical to set ORG before spitting any-
+thing. Boot binaries will usually take care of this, so you
+don't have to do it yourself. You just have to make sure that
+you load the boot binary right after loading B270.
+
+Then, you spit your content following instructions in
+bootstrap.txt.
+
+After you're done, you can run EMPTY to go back to a usable
+system.