Your thoughts please - bitwise shift operation

[Anobium]

Help and comments please.

I need help to get this right. The change to the compiler is actually very small. But, getting the syntax right is very critical.

The other day I was contact regarding porting LATA ^=(1 << 6); to GCBASIC. This is a question I have seen many times.

This operation equates to LATA = LATA XOR 64 but we have lots of options in GCBASIC ( and, I am sure there are more ways to do this) but none are an like for like implementation.

All of the following work - the first two are optimised but all but the last option change the context of the shift operation.

Instruction	ASM Size	Comment
PORTA = PORTA xor 64	2	The user have to calculate the bit value ( 64 )
PORTA = PORTA XOR 0b01000000	2	The user have to calculate the bit value ( 64 )
PORTA.6 = NOT PORTA.6	6	Logically correct and easy to use
PORTA.6 = PORTA.6 XOR 1	8	This option is useful but code is large.
SetWith( PORTA.6, !PORTA.6 )	9	The oldest GCBASI C way!!!
PORTA = PORTA XOR FnLSR ( 1, 6 )	19	A true maths conversion... the slowest to execute.

The GCBASIC compiler really should convert (1 << 6) as follows

#chip 16F877a
#option Explicit

#DEFINE myShift 4  // could be any value constant/register mask. I am using the constant value of 4

Do
    porta = porta XOR ( 1 << myShift )
    wait 100 ms
Loop

Explanation:

porta is typically a register that controls the state of the pins on port A of the microcontroller.

XOR (exclusive OR) is a bitwise operation that toggles specific bits.

1 << myShift: This is a bitwise left shift operation. It shifts the bit 1 to the left by myShift positions. So 1 << 4 equals 16 in decimal, or 0b10000 in binary.

The XOR operation will toggle the bit at position 4 of register.

ASM

The ASM generated follows. It is optimised and is easy to understand.

Instruction	ASM Size	Comment
porta = porta XOR ( 1 << myShift )	2	Simple and fast.

This is generated by my prototype implementation.

   ;porta = porta XOR ( 1 << myShift )
    movlw   16
    banksel PORTA
    xorwf   PORTA,F

---

Syntax and Error Handling

result_register = ( VALUE_CONSTANT [<< | >> ] NUM_OF_POSITIONS_CONSTANT )

[EDITED] The shift operations would have to support constants only. There is huge difference between using constants to variables. Constants can be calculated during compilation. Variables are a run time coding and is not scope for this potential piece of work.

See next post regarding variables as parameters

This seems to be so easy but this where I need your thoughts to get the syntax right.

How to handle errors? what are errors? etc.
Is this valid syntax ? porta = porta XOR 1 << myShiftNo parentheses
How handle ? porta = porta XOR 1 << Where there is no shift parameter
How handle ? porta = porta XOR 1 << undefined_consant Where there is no shift parameter as the constant does not exist.
How to handle ? Using a variable as a shift parameter? Just throw a compilation error.
Other things that need to be handled ?

---

Evan

[Anobium]

Change of plan ... as I posted a private message to Chris Roper ( he wrote many of the Bitwise operations many years ago... I had a flash of inspiration.

Support of variables as the parameters is easily do able. If either of the parameters are not constants then call then transform the instruction ( in the compier) to call methods FnLSL() or FnLSR() within the compiled ASM - the user source remains the same!

**Prototype Compiler Implementation **

Dim myShiftVar as Byte
myShiftVar = 4

Do
    porta = porta XOR ( 1 << myShiftVar )
    wait 100 ms
Loop

Generated ASM!

;porta = porta XOR ( 1 << myShiftVar )
    movlw   1
    movwf   SYSBYTETEMPB
    movlw   4
    banksel NUMBITS
    movwf   NUMBITS
    call    FN_FNLSL91
    movf    PORTA,W
    xorwf   SYSFNLSLBYTE,W
    movwf   PORTA

...
...

;Overloaded signature: BYTE:BYTE:, Source: stdbasic.h (427)
FN_FNLSL91
;Repeat NumBits
    movf    NUMBITS,W
    movwf   SysRepeatTemp1
    btfsc   STATUS,Z
    goto    SysRepeatLoopEnd1
SysRepeatLoop1
;Set C Off
    bcf STATUS,C
;Rotate SysByteTempB Left
    rlf SYSBYTETEMPB,F
;End Repeat
    decfsz  SysRepeatTemp1,F
    goto    SysRepeatLoop1
SysRepeatLoopEnd1
;FnLSL = SysByteTempB
    movf    SYSBYTETEMPB,W
    movwf   FNLSL
    return

Crikey - this works!!

[Anobium]

I have completed an implementation.

Handles use of constants or variables.

porta =  porta XOR 1 << 6

Gens

;porta =  porta XOR ( 1<<6 )
    movlw   64
    banksel PORTA
    xorwf   PORTA,F

or

Dim Numbits as Byte
Numbits = 6
porta =  porta XOR 1 << Numbits

Gens

;Dim Numbits as Byte
;Numbits = 6
    movlw   6
    banksel NUMBITS
    movwf   NUMBITS
;porta =  porta XOR 1 << Numbits
    movlw   1
    movwf   SYSBYTETEMPB
    call    FN_FNLSL91       // Call to FNLSL or FNLSR as appropriate
    movf    PORTA,W
    xorwf   SYSFNLSLBYTE,W
    movwf   PORTA

There is lots of syntax checking but it may not cover all cases.

Anyone interested in testing?