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:

        sta $1900
        stx $1901
        sty $1902

        ldx #54
.loop   sta $1900,X
        dex
        bne loop

        lda #0
        sta $70
        lda #$20
        sta $71
        ldy #0
        sta ($70),Y

        lda #0
        sta $70
        lda #$20
        sta $71
        ldx #0
        sta ($70,X)