gforth: gforth/model

1 :	anton	1.1	$Id: model,v 1.3 1992/09/11 12:13:54 anton Exp $
2 :			Copyright 1992 by the ANSI figForth Development Group
3 :
4 :			This file describes the implementation model of ANSI figForth. The things
5 :			described herein are valid in all implementations of ANSI figForth.
6 :
7 :			The instruction pointer (ip) is post-incremented.
8 :
9 :			Branches are relative:
10 :			branch
11 :			offset: target - offset
12 :			...
13 :			target: ...
14 :
15 :			The top-of return stack is in memory
16 :
17 :			The top of data stack need not be in memory. Other data stack items
18 :			are in memory.
19 :
20 :			The stacks grow downward
21 :
22 :			flags are represtented by 0 and -1
23 :
24 :			The loop control parameters are the index on the top of return stack
25 :			and the limit on the second item.
26 :
27 :			Everything except the virtual machine registers (ip, sp, etc.) is
28 :			addressable.
29 :
30 :			The return address on the return stack occupies one cell.
31 :
32 :			There is a floating point stack
33 :			The floating point numbers are the same as C's double numbers
34 :
35 :			The error returns of the file word set are FALSE or TRUE. (or should
36 :			we use errno? Does this work with stdio functions?)
37 :
38 :			No assumptions can be made about the order of fetching the xt of the
39 :			next word wrt the other operations in a forth word. This makes a
40 :			difference only if you want to execute code lying on the stack or if
41 :			you store into the very next word that is executed.