Problem Statement

In systems programing it is sometimes required to place language objects at specific memory locations, like I/O registers, hardware interrupt vectors etc.

Show how language objects can be allocated at a specific machine addresses. Since most OSes prohibit access to the physical memory if it is not mapped by the application, as an example, rather than a physical address, take the address of some existing object (using suitable address operations if necessary).

For example:

Let's start with the solution:

type
  MyObject = object
    x: int
    y: float

var
  mem = alloc(sizeof(MyObject))
  objPtr = cast[ptr MyObject](mem)
echo "object at ", cast[int](mem), ": ", objPtr[]

objPtr[] = MyObject(x: 42, y: 3.1415)
echo "object at ", cast[int](mem), ": ", objPtr[]


var x: int = 3
var p: ptr int

p = cast[ptr int](addr(x))

echo "Before ", x
p[] = 5
echo "After: ", x