Dear Roman Black,

Roman Black <fastvid@EZY.NET.AU> on 2001-05-30 12:13:27 PM asked,
...
>a 12-core PIC, you can only put calls and
>computed gotos on the bottom 256byte blocks. Those
>are all full of other tables. I need a sane way
>of adding a small table in another area (a high
>block where tables are not possible). I can do it
>with the 4:1 system I showed but that is very
>unpolished. Surely someone has done this before?
>:o)
>-Roman

Hm, interesting challenge. How about something like

  ; warning, untested code by david cary
   UDATA
i res 1
  ...
  CODE
  ; somewhere in the bottom of a 256 byte block
lookup:     ; just 1 instruction needed in LO memory
  goto real_table_start
  ...
  call lookup ; w = table[i].
  ; now table[i] is in w, or if i was out of range, the default value is in w.
  ...
  ; OK to put in HI memory. (Also OK to put in LO memory, but then, what's the
point ?)
real_table_start:
  tstf i
  skpnz
  retlw Z ; for 0 == i
  decf i
  skpnz
  retlw A ; for 1 == i
  decf i
  skpnz
  retlw B ; for 2 == i
  decf i
  skpnz
  retlw C ; for 3 == i
  retlw default_value ; catches all other values of i.

I count 15 instructions / 5 values = 3 instructions/value. This looks easy to
extend to 256 values.

Here's something a little more compact, but it is limited to 9 possible values,
and the indexing is a little odd:

  ; warning: untested code by david cary
real_table_start:
  btfsc i,7
    retlw A ; for 0x80 <= i <= 0xFF
  btfsc i,6
    retlw B ; for 0x40 <= i < 0x80
  btfsc i,5
    retlw C ; for 0x20 <= i < 0x40
  btfsc i,4
    retlw D ; for 0x10 <= i < 0x20
  btfsc i,3
    retlw E ; for 8 <= i < 0x10
  btfsc i,2
    retlw F ; for 4 <= i < 8
  btfsc i,1
    retlw G ; for 2 <= i < 4
  btfsc i,0
    retlw H ; for 1 == i
    retlw J ; for 0 == i.

If some of your large tables are "smooth", you might be able to compress them,
then (at runtime) use interpolation to approximate the missing values. This is a
classic time/space tradoff. It's generally easier to find some way to compress a
big block of data than to try to individually compress little pieces one at a
time.

--
David Cary

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body