🚀 go-pugleaf

In article <30sis-c@massey.ac.nz> G.Moretti@massey.ac.nz writes:

>Take a look at the following file.  If I remove the ORG $0000 the errors
>go away.  However this sort of erroneous behaviour in an assembler makes
>it unusable, especially when if you put the MSG on a line by itself the
>PHASING ERROR message goes away, leaving an apparently error-free assembly.
>------------------------------------------------------------------------------
>0001 0100                             org  $100
>0002
>0003 0100 ce 01 0e                    ldx  #msg        <---- Address is wrong
>0004 0103 9d 00                       jsr  led_on
>0005 0105 20 04                       bra   done       <---- Should be  20 03
>0006 0107 c3 00 01                    addd #1
>0007 010a 7e 01 0a           done     jmp   done
>0008
>9| Phasing Error
>0009 010d 48 65 6c 6c 6f     msg      fcc  "Hello"
>0010                         ************************************************
>0011 0000                             org  $0000  <---- remove this - all ok
>0012
>0013 0000 b6 10 03           led_on   lda  $1003
>0014 0003 8a 80                       ora  #$80
>0015 0005 b7 10 03                    sta  $1003
>0016 0008 39                          rts
>----------------------------------------------------------------------------

Giovanni,

The phasing error is due the composition of two things:

(1) your code makes forward references to labels;

(2) later in the assembly stream, code is generated that resides
earlier in the memory of the 6811.  

Because AS11 makes only two passes on the code, it gets confused.  
This is a known problem (the phasing error) and is discussed in the
documentation for the AS11 assembler (the file ASEMBLER.DOC).

Here's what occured for your sample code: In the first pass, the
assembler has to figure out how large the arguments are for each
instruction so that it can resolve forward label references.  The
trouble is with the label 'led-on'.  In the first pass, the assembler
assumes the "JSR led-on" will require three bytes (one byte
instruction, two bytes data).  But then it discovers (after your ORG
$0000 puts 'led-on' in the zero page) that the 'led-on' label can be
expressed in one byte.  When it goes to fill in the label values, it
seems to optimize this value to fit into one byte, generating the
error.

If you structure your assembly source code so that all code is
generated in increasing order of location in the 6811 memory map, AS11
should do fine.

The only other 6811 assembler I've used is *not* freeware, but it does
deal with this problem by having the capability to make an arbitrary
number of passes on the source code until forward references and code
placement are resolved.  It's named ASM11 and is sold by Dunfield
Development Systems, Inc. (phone: 613-256-5820).

	- Fred Martin