sdc: don't use ABORT"

ABORT" doesn't work in xcomp.

Also, improve xcomp docs and fix misdocumentation.

blk/430

@@ -1,4 +1,4 @@
-: _err 0 (spie) ABORT" SDerr" ;
+: _err 0 (spie) LIT" SDerr" ERR ;
 
 ( Tight definition ahead, pre-comment.
 

doc/cross.txt

@@ -71,3 +71,28 @@ bootstrap.txt.
 
 After you're done, you can run EMPTY to go back to a usable
 system.
+
+# Immediate compiling words trickyness
+
+When using an immediate compiling word such as "IF" during
+xcomp, things are a bit tricky for two reasons:
+
+1. Immediates used during xcomp are from the host system.
+2. The reference of the word(s) they compile is for the host
+   system.
+
+Therefore, unless the compiled word (for example (?br) compiled
+by IF) has exactly the same address in both the host and guest,
+the resulting binary will be broken.
+
+For this reason, we re-implement many of those compiling words
+in xcomp overrides, hacking our way through, so that those
+compiling words compile proper guest references. We don't do
+this for all compiling words though. This means that some words
+can't be used in core and drivers, for example, ABORT" and .".
+
+How to know whether a word can be used?
+
+1. If it's not an immediate compiling word, it's fine.
+2. If its overriden in B270, it's fine.
+3. Otherwise, you can't cross-compile it.

recipes/rc2014/blk/606

@@ -1,5 +1,5 @@
 Zilog SIO driver
 
-Declarations at B587
+Declarations at B607
 
-Driver load range at B588-B590
+Driver load range at B608-B610

recipes/rc2014/sdcard.md

@@ -84,7 +84,7 @@ and `SPI_CTL`,  which are respectively `4` and `5` in our relay design.
 You also need to tell the SDC subsystem which SPI device to activate by defining
 the `SDC_DEVID` (1, 2, 4, 8 for device 0, 1, 2 or 3)
 
-You can then load the driver with `596 LOAD`. This driver provides
+You can then load the driver with `616 LOAD`. This driver provides
 `(spix)` and `(spie)` which are then used in the SDC driver.
 
 The SDC driver is at B420. It gives you a load range. This means that what