Memory allocation: Difference between revisions

 

The second approach, suitable especially if you want to reserve a few bytes for a small machine code subroutine, is to hide the storage space you want inside a comment. When you enter a <code>REM</code> statement, the interpreter sets aside sufficient bytes to store the text of your comment <i>and then doesn't care what you put in them</i>: if you know the address where the comment is stored, therefore, you can <code>POKE</code> whatever values you like into that space. If the comment is the first line in the program, the <code>REM</code> itself will be at address 16513 and the comment text will begin at 16514 and take up one byte per character. An example, with a trivial machine code routine (it adds 3 and 4):

30 LET ADDR=16514

40 POKE ADDR,CODE P$*16+CODE P$(2)-476

80 CLEAR

90 PRINT USR 16514</lang>

<lang z80asm>3e 03 ld a, 3

81 add a, c

4f ld c, a

c9 ret</lang>

Line <tt>40</tt> reads a two-digit hex number and pokes its value into memory. <code>USR</code> ("user sub routine"), in line <tt>90</tt>, is a function that takes the address of a machine language routine, calls it, and returns the contents of the <tt>BC</tt> register pair when the routine terminates. Under normal circumstances, once you were satisfied the machine code program was working correctly you would remove lines <tt>20</tt> to <tt>80</tt>, leaving just the machine code subroutine and the call to it. Note that if you list the program once you have run it, the first line will look something like this:

Unfortunately, you cannot type that in directly: not all 256 possible values are accessible from the keyboard, so there is no point trying to learn to enter machine code in that form.