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New section about porting

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git-svn-id: svn://svn.cc65.org/cc65/trunk@2452 b7a2c559-68d2-44c3-8de9-860c34a00d81
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cuz 2003-09-27 14:42:24 +00:00
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doc/ca65.sgml
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@ -561,8 +561,8 @@ names like "Loop". Here is an example:
dey
bne @Loop ; Ok
rts
Sub: ... ; New global label
bne @Loop ; ERROR: Unknown identifier!
Sub: ... ; New global label
bne @Loop ; ERROR: Unknown identifier!
</verb></tscreen>
<sect1>Unnamed labels<p>
@ -2934,7 +2934,90 @@ specify an additional type. Predefined types are 0 (constructor) and 1
</itemize>
<sect>Porting sources from other assemblers<p>
Sometimes it is necessary to port code written for older assemblers to ca65.
In some cases, this can be done without any changes to the source code by
using the emulation features of ca65 (see <tt><ref id=".FEATURE"
name=".FEATURE"></tt>). In other cases, it is necessary to make changes to the
source code.
Probably the biggest difference is the handling of the <tt><ref id=".ORG"
name=".ORG"></tt> directive. ca65 generates relocatable code, and placement is
done by the linker. Most other assemblers generate absolute code, placement is
done within the assembler and there is no external linker.
In general it is not a good idea to write new code using the emulation
features of the assembler, but there may be situations where even this rule is
not valid.
<sect1>TASS<p>
You need to use some of the ca65 emulation features to simulate the behaviour
of such simple assemblers.
<enum>
<item>Prepare your sourcecode like this:
<tscreen><verb>
; if you want TASS style labels without colons
.feature labels_without_colons
; if you want TASS style character constants
; ("a" instead of the default 'a')
.feature loose_char_term
.word *+2 ; the cbm load address
[yourcode here]
</verb></tscreen>
notice that the two emulation features are mostly useful for porting
sources originally written in/for TASS, they are not needed for the
actual "simple assembler operation" and are not recommended if you are
writing new code from scratch.
<item>Replace all program counter assignments (which are not possible in ca65
by default, and the respective emulation feature works different from what
you'd expect) by another way to skip to another memory location, for example
the <tt><ref id=".RES" name=".RES"></tt>directive.
<tscreen><verb>
; *=$2000
.res $2000-* ; reserve memory up to $2000
</verb></tscreen>
notice that other than the original TASS, ca65 can never move the
programmcounter backwards - think of it as if you are assembling to disc with
TASS.
<item>Conditional assembly (<tt/.ifeq//<tt/.endif//<tt/.gogo/ etc.) must be
rewritten to match ca65 syntax. Most importantly notice that due to the lack
of <tt/.goto/, everything involving loops must be replaced by
<tt><ref id=".REPEAT" name=".REPEAT"></tt>.
<item>To assemble code to a different address than it is executed at, use the
<tt><ref id=".ORG" name=".ORG"></tt> directive instead of
<tt/.offs/-constructs.
<tscreen><verb>
.org $1800
[floppy code here]
.reloc ; back to normal
</verb></tscreen>
<item>Then assemble like this:
<tscreen><verb>
cl65 --start-addr 0x0ffe -t none myprog.s -o myprog.prg
</verb></tscreen>
notice that you need to use the actual start address minus two, since two
bytes are used for the cbm load address.
</enum>
<sect>Bugs/Feedback<p>