FAST Forth Native-Language Embedded Computers

forward branches (such as those
generated by the if ... then
control structure) and to back-
ward branches (such as loops).
Figure Five illustrates several
examples of rectifying the stack
state and the value of Voffs at
branch targets.
2. All compiled Forth words must
be able to assume that the stack
state and Voffs value are set to
default values upon entry. This
means that a calling word must
enforce those defaults before
calling another compiled word.
Likewise, a calling word needs
to be able to assume that those
same defaults are present on
return; the called word must
enforce the defaults upon exit-
ing. Coercion to defaults does
not imply a restriction on the
usage of the data stack as seen
by the Forth programmer; it
does, however, imply that ex-
tra code to set up the stack state
and the Voffs value may need
to be gen~rated.~
3. If the stack pointer needs to be
accessed (via the word @ sp or a
similar mechanism), it must be
derived by taking the value of
the data-stack pointer EDI, then
modifying it to account for both
the stack state and the current
value of Voffs. Likewise, if it is
expected that items on the stack
will be made available for
memory accesses, then the con-
tents of all registers that buffer
data-stack items (EAX/EBX/
EDX) must be forced out to
memory prior to any such ac-
cesses; this can be accomplished
by rectifying the stack to state 0.
Figure Two-b. swap's generated code and change-of-stack-state. 1
EDX 3rd i tem ------------
[ED11 4th item
[ED1+4] 5th item
[ED1+8] 6th item
-
[EDI+12] 7th item
The rectification of one stack state to another is
accompanied by the generation of code that moves values
between the EAX/EBX/EDX registers and, possibly, the
Top item
Figure Two-c. xor's generated code and change-of-stack-state.
2nd item
EDX 3rd item
Starting stack state = 1 Stack state is unchanged
memory portion of the data stack; the value of Voffs may
be modified in the process. Rectification of the value of
Voffs to another value Voffs(,,,) involves generating code
that adds the difference V~ffs(,,,,,~ -Voffs(,,,) to the datastack
pointer EDI, then setting Voffs to Voffs(,,,). If
rectification does not involve a change to either the Current
4The same considerations for calling a compiled Forth word also apply to the
use of execute to call a word indirectly via an executable addresson the data
-
stack. In add~tion, Forth code should neverattempt toenterany compiled Forth stack state or the Voffs value, no code is generated.
word, except at the legitimate entry point at the start of the word.
Some early Forth programs take advantage of the fact that many Forth
implementationscompileeachnewwordintheformofasequenceofaddresses
that point to the called words. Since an increasing number of Forth
implementations generate machine code directly, such techniques should be
completely avoided. Instead, if a table of executable-word addresses is
needed, the table should be built explicitly as an array of addresses that can
be used with the execute word. (The ANSI Forth standard recognizes that
different Forth implementations mav use comoletelv different methods to
March 1994 April
. ,
Starting stack state = 12
generate and interpret run-time code'; it specifically prohibits "tricks," such as
the one just mentioned, that may be dependent on the way in which Forth code
is generated.)
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It is important to remember that both the stack state and
the Voffs value are significant only during compilation.
while the resulting generated code will reflect the influenceS
of those there is no run-time tracking of what
those values were in the generated code. (However, any
data structures generated for debugging purposes will
need to track those values in order to disda~ the data
stack's Contents correctly,)
1
[ED11 4th item
[ED1+4] 5th item
[ED1+8] 6th item
[ED1+12] 7th item
J. New stack state = 10
Forth Dimensions