Revision as of 05:04, 22 April 2011

A simple graphics language for microcontrollers

We start with a very simple reverse polish notation interpreter with a preprocessor stage

Sample code:

to circle 360 for 1 forward 1 right next end

Registers:

0 X (nn.ff)
1 Y
2 Angle
3 Scale
4 Colour
5 Pen up/down
6 Nested If counter
7

Stack diagrams

( things read from the stack -- things written on the stack )
x - x position
y - y position
n - number
Example:
  ( n n -- n )
  This means that two numbers were read from the stack, then one number was written back.
  For example a simple addition reads two numbers and writes back the result.

  ( a b c -- c b a )
  Reading threee values from the stack and write them back in the opposite order.
 

Definitions

• Function:
  • To < function name > < code > End
• Constant:
  • Define < constant name > < costant > End
• Macro:
  • Macro < macro name > < code > End

Commands

Absolute commands
  setcolour     ( colour -- )
  readcolour    ( -- colour )
  setpos        ( x y -- )
  readpos       ( -- x y )
  load          ( address -- data )
  store         ( data address -- )

Relative commands:
  penup         ( -- )
  pendown       ( -- )
  forward       ( length -- )
  backward      ( length -- )
  left          ( angle -- )
  right         ( angle -- )
 
Loops:
  for           ( n -- )
  next          ( -- )
  do            ( -- )
  again         ( -- )

Conditionals:
  if            ( n -- )
  else          ( -- )
  endif         ( --  )

Data processing:
  +             ( n n -- n )        ( to +  >r 0 r> - - end )
  -             ( n n -- n )        ( fundamental )
  *             ( n n -- n )        ( to * 0 do over 1 and if rot dup >r -rot r> + endif rot 1 lsl rot 1 lsr rot over if again endif >r drop -rot drop drop end ) 
  /             ( n n -- n )
  lsl           ( n n -- n )        ( to lsl for dup + next end )
  lsr           ( n n -- n )        ( to lsr for 31 for dup 0x80000000 and swap 1 lsl swap if 1 + endif next 0x7fffffff and next end )
  and           ( n n -- n )        ( fundamental )
  or            ( n n -- n )        ( to or over not and + end )
  eor           ( n n -- n )        ( to eor over over or -rot and not and end )
  not           ( n -- n )          ( to not >r -1 r> - end )

Stack manipulation:
 drop           ( n -- )           ( to drop if endif end )
 dup            ( n -- n n )       ( fundamental )
 swap           ( a b -- b a )     ( to swap dup >r >r dup r> eor dup >r eor r> r> eor end )
 over           ( a b -- a b a )   ( to over >r dup r> swap end ) 
 rot            ( a b c -- b c a ) ( to rot >r swap r> swap end ) 
 -rot           ( a b c -- c a b ) ( to -rot swap >r swap r> end )
 >r             ( n -- )           ( fundamental )
 r>             ( -- n )           ( fundamental )

Optimal stack reordering in Forth

The dictionary

The dictionary used to convert a command into an opcode. It is a zero terminated string like "setcolour readcolour setpos readpos ",0 that is searched linearly from start to end. A space marks the end of a word. The first compiler pass adds all functions to the end of this table.

The function jump table

This table contains the address of each function, in the same order as the functions appear in the dictionary. It is filled in during the second pass when the code position is known.

Interpreter

do
  read opcode
  if number then add number to the data stack
  if hardwired command then execute
  if function then look up the address in the function jump table and call it
again

Opcodes:
0 load 0
1 load next byte on to the stack
2 load the next two bytes
3 load the next three bytes
4 load the next four bytes
5 execute in line THUMB2 code
6-15 reserved
16-255 commands