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Fast Looping Intrinsics

The following words may be used to produce faster loops. They remove the need for explicit comparisons prior to the exit condition (UNTIL). Tests (in short, tight loops) have shown them to be from 25 to 30% faster. Of course, the more complex the code in the loop, the less the payoff.

The words below were written with the Forth 9900 Assembler, available on block 9 of the boot disk, then converted to machine CODE words using the ASM>CODE utility on block 29 of the boot disk. For anyone interested, the Forth assembler source is also given. Note: Using the CODE words means you don't need to load the assembler to use the words.

Common Code

The following code is common to all the routines below. Since TurboForth checks that all BEGINs have a matching UNTIL, a hack is required to adjust the reference counting:

: doRefCnt ( -- ) \ handle BEGIN reference counting -1 $A086 +! ;

LOOP=0 ( -- )

Loops back to preceding BEGIN if TOS=0. TOS is not consumed.

Size: 46 bytes.

CODE: (loop=0) C033 C054 1601 C0C0 ;CODE : loop=0 ( beginAddr -- ) \ needs address laid down by BEGIN compile (loop=0) , \ compile call and loop-back address doRefCnt \ handle BEGIN reference counting ; immediate

Test Code

: test ( n -- ) begin dup . 1- loop=0 ; 50 test

The loop will not execute since TOS is not zero.

Equivalent Forth Assembler Code

ASM: (loop=0) ( -- ) \ loop while TOS=0. TOS is not consumed.
    \ the loop back address is in-line. Can be accessed via PC (R3)
    r3 *+ r0 mov,           \ get loop-back address and advance PC
    *sp r1 mov,             \ test TOS
    eq if,
        r0 r3 mov,          \ load program counter with loop-back address 
    endif,
;ASM 

LOOP<>0 ( -- )

Loops back to preceding BEGIN if TOS<>0. TOS is not consumed.

Size: 50 bytes.

CODE: (loop<>0) C033 C054 1301 C0C0 ;CODE : loop<>0 ( beginAddr -- ) \ needs address laid down by BEGIN compile (loop<>0) , \ compile call and loop-back address doRefCnt \ handle BEGIN reference counting ; immediate

Test Code

: test ( n -- ) begin dup . 1- loop<>0 ; 50 test

The loop will execute since TOS is not zero.

Equivalent Forth Assembler Code

ASM: (loop<>0) ( -- ) \ loop while TOS <>0. TOS is not consumed.
    \ the loop back address is in-line. Can be accessed via PC (R3)
    r3 *+ r0 mov,           \ get loop-back address and advance PC
    *sp r1 mov,             \ test TOS
    ne if,
        r0 r3 mov,          \ load program counter with loop-back address 
    endif,
;ASM

LOOP>0 ( -- )

Loops back to preceding BEGIN if TOS>0. TOS is not consumed.

Size: 48 bytes.

CODE: (loop>0) C033 C054 1501 1001 C0C0 ;CODE : loop>0 ( beginAddr -- ) \ needs address laid down by BEGIN compile (loop>0) , \ compile call and loop-back address doRefCnt \ handle BEGIN reference counting ; immediate

Test Code

: test ( n -- ) begin dup . 1- loop>0 ; 50 test

The loop will execute until TOS=0.

Equivalent Forth Assembler Code

ASM: (loop>0) ( -- ) \ loop while TOS>0. TOS is not consumed.
    \ the loop back address is in-line. Can be accessed via PC (R3)
    r3 *+ r0 mov,           \ get loop-back address and advance PC
    *sp r1 mov,             \ test TOS
    gt if,
        r0 r3 mov,          \ load program counter with loop-back address 
    endif,
;ASM

LOOP<0 ( -- )

Loops back to preceding BEGIN if TOS<0. TOS is not consumed.

Size: 48 bytes.

CODE: (loop<0) C033 C054 1101 1001 C0C0 ;CODE : loop<0 ( beginAddr -- ) \ needs address laid down by BEGIN compile (loop<0) , \ compile call and loop-back address doRefCnt \ handle BEGIN reference counting ; immediate

Test Code

: test ( n -- ) begin dup . 1+ loop<0 . ; -50 test

The loop will execute until TOS is no longer negative.

Equivalent Forth Assembler Code

ASM: (loop<0) ( -- ) \ loop while TOS<0. TOS is not consumed.
    \ the loop back address is in-line. Can be accessed via PC (R3)
    r3 *+ r0 mov,           \ get loop-back address and advance PC
    *sp r1 mov,             \ test TOS
    lt if,
        r0 r3 mov,          \ load program counter with loop-back address 
    endif,
;ASM

LOOP= ( a b -- )

Loops back to preceding BEGIN if a=b. b is consumed.

Size: 48 bytes.

CODE: (loop=) C033 C074 8054 1601 C0C0 ;CODE : loop= ( beginAddr -- ) \ needs address laid down by BEGIN compile (loop=) , \ compile call and loop-back address doRefCnt \ handle BEGIN reference counting ; immediate

Test Code

: test ( a -- ) begin dup . 1+ 5 ( b) loop= . ; 0 test

The loop will not execute because a and b are not equal.

Equivalent Forth Assembler Code

ASM: (loop=) ( n -- ) \ loop while TOS=n. TOS is not consumed.
    \ the loop back address is in-line. Can be accessed via PC (R3)
    r3 *+ r0 mov,           \ get loop-back address and advance PC
    *sp+ r1 mov,            \ get n
    *sp r1 cmp,             \ compare n and TOS
    eq if,
        r0 r3 mov,          \ load program counter with loop-back address 
    endif,
;ASM

LOOP<> ( a b -- )

Loops back to preceding BEGIN if a <>b. b is consumed.

Size: 48 bytes.

CODE: (loop<>) C033 C074 8054 1301 C0C0 ;CODE : loop<> ( beginAddr -- ) \ needs address laid down by BEGIN compile (loop<>) , \ compile call and loop-back address doRefCnt \ handle BEGIN reference counting ; immediate

Test Code

: test ( a -- ) begin dup . 1+ 5 ( b) loop<> . ; 0 test

The loop executes until a and b are equal.

Equivalent Forth Assembler Code

ASM: (loop<>) ( a b -- ) \ loop while TOS<>n. TOS is not consumed.
    \ the loop back address is in-line. Can be accessed via PC (R3)
    r3 *+ r0 mov,           \ get loop-back address and advance PC
    *sp+ r1 mov,            \ pop b
    *sp r1 cmp,             \ compare a and b
    ne if,
        r0 r3 mov,          \ load program counter with loop-back address 
    endif,
;ASM 

30th May 2016

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