Constant expressions

[GTBecker]

Is an expression that will always yield a constant evaluated at run time or it is equivalent to a constant?

For example, is there a price for the following self-documenting expression?

Code: Select all

	case asc("0")

[dkinzer]

Is an expression that will always yield a constant evaluated at run time or it is equivalent to a constant?

Unless you turn off optimization (which is on by default) the compiler will detect compile-time constant value expressions (including those involving ZBasic System Library functions) and substitute the constant value. Consider this simple example:

Code: Select all

Dim b as Byte
Sub Main()
  Select Case (b)
  Case Asc("0")
    Debug.Print "xx"
  End Select
End Sub

For a VM target, the resulting code looks like this:

Code: Select all

            Sub Main()
            	Select Case (b)
0019 1d4001      PSHA_B         0x0140 (320)
001c 1a30        PSHI_B         0x30 (48)
001e a0          CMPEQ_B
001f 030300      BRA_T          0025
0022 010800      BRA            002d
            	Case Asc("0")
            		Debug.Print "xx"
0025 09027878    PSHI_S         "xx"
0029 fe26        SCALL          OUTSTR
002b fe27        SCALL          OUTEOL
            	End Select
            End Sub
002d 06          RET

Given that the result of Asc("0") is the value &H30, you can see that the instructions at 001c and 001e compare that value to the selection expression.

In contrast, with optimization off the following code is produced:

Code: Select all

            Sub Main()
            	Select Case (b)
0019 1d4001      PSHA_B         0x0140 (320)
001c 090130      PSHI_S         "0"
001f 1b0100      PSHI_W         0x0001 (1)
0022 fe1b        SCALL          STRASC
0024 a0          CMPEQ_B
0025 030300      BRA_T          002b
0028 010800      BRA            0033
            	Case Asc("0")
            		Debug.Print "xx"
002b 09027878    PSHI_S         "xx"
002f fe26        SCALL          OUTSTR
0031 fe27        SCALL          OUTEOL
            	End Select
            End Sub
0033 06          RET

Here you can see that the string "0" is pushed and the system call STRASC is invoked.

Even though the mnemonics of the VM instructions have not been published, I think that it is not too difficult to get the gist of the code.

[mikep]

It is worth noting that the ZBasic compiler can handle quite sophisticated expressions. If you are in doubt you can always declare a constant and see if the compiler complains. For example

Code: Select all

Private Const Expr1 as Byte = Asc("0")

[GTBecker]

Very nice, guys; thanks.

BTW, FWIW, here's a MAX7456 OSD ( http://www.sparkfun.com/commerce/produc ... ts_id=9168 ) on a ZX-24, with an NTSC IR camera. The 7456 is both providing the sync pulse for timing and allows putting resulting hotspot vector data, not yet in code, on the image:

http://www.ustream.tv/channel/ircam