FreeBasic Bare Bones: Difference between revisions

 

In FreeBasic, a pointer is defined by adding the Ptr keyword.

Dim mypointer As Byte Ptr

</syntaxhighlight>

This one holds the location of a byte of memory.

Dim mypointer As Long Ptr

</syntaxhighlight>

You can not use pointers straight away. Consider sending a letter with no address on it will not do any good. So we must first put an address in there. That leads to the question: how do we get an address. Some things have fixed addresses, like the video card.

You can also ask variables for their addresses.

Dim variable as Byte

Dim pointer as Byte Ptr

</syntaxhighlight>

The @ returns the address of the variable that follows it.

Dim pointer as Byte Ptr

Dim address as Long

</syntaxhighlight>

Here CPtr (Convert to Pointer) is used to create pointers. You give it a type (Byte Ptr) and the address. You can also change pointers:

Dim pointer1 as Byte Ptr

Dim pointer2 as Long Ptr

 

You can access pointers with an index:

Dim value as Long

Dim pointer as Long Pointer

 

Strings are useful, but tedious. A string is stored as a series of bytes in consecutive addresses. You can build a pointer to a string as well:

Dim s as String

Dim pointer as String Ptr

</syntaxhighlight>

But since we do not have the Runtime, we have to use something that does not use it.

Const s = "Text"

Dim pointer as Byte Ptr

Basic uses an borrowed trick to tell us the end of the string. After the last character, there will always be a 0. So if we read a string in order, we can tell when it has ended.

 

Sub PrintString(src As Byte Ptr, x As Long, y As Long)

 

=== link.ld ===

OUTPUT_FORMAT("elf32-i386")

ENTRY (loader)

}

}

 

=== Build instructions ===

 

<tt>kernel.bin</tt> can then be loaded by [[GRUB]]