Editing Data General Nova (section)

===Binary print accumulator===
The following routine prints the value of AC1 as a 16-digit [[binary number]], on the RDOS console. It reveals further quirks of the Nova instruction set. For instance, there is no instruction to load an arbitrary "immediate" value into an accumulator (although memory reference instructions do encode such a value to form an effective address). Accumulators must generally be loaded from initialized memory locations (e.g. <code>n16</code>). Other contemporary machines such as the [[PDP-11]], and practically all modern architectures, allow for immediate loads, although many such as [[ARM architecture family|ARM]] restrict the range of values that can be loaded immediately.

Because the RDOS <code>.systm</code> call macro implements a <code>jsr</code>, AC3 is overwritten by the return address for the <code>.pchar</code> function. Therefore, a temporary location is needed to preserve the return address of the caller of this function. For a recursive or otherwise re-entrant routine, a stack, hardware if available, software if not, must be used instead. The return instruction becomes <code>jmp @ retrn</code> which exploits the Nova's indirect addressing mode to load the return PC.

The constant definitions at the end show two assembler features: the assembler radix is [[octal]] by default (<code>20</code> = sixteen), and character constants could be encoded as e.g. <code>"0</code>.
<syntaxhighlight lang="nasm">
 pbin:  ; print AC1 on console as 16 binary digits, by Toby Thain

        sta     3,retrn     ; save return addr
        lda     2,n16       ; set up bit counter
 loop:  lda     0,chr0      ; load ASCII '0'
        movzl   1,1,szc     ; get next bit in carry
        inc     0,0         ; bump to '1'
        .systm
        .pchar              ; AC0-2 preserved
        jmp     err         ; if error
        inc     2,2,szr     ; bump counter
        jmp     loop        ; loop again if not zero
        lda     0,spc       ; output a space
        .systm
        .pchar
        jmp     err         ; if error
        jmp     @retrn

 spc:   " ;that's a space
 chr0:  "0
 n16:   -20
 retrn: 0
</syntaxhighlight>