New 211BSD idle pattern

[Jeff Thieleke]

I never was impressed with the basic 211BSD idle pattern (especially compared to some of the other operating system's patterns), so I made small improvement to the default.  It is now a "Knight Rider" style of left to right and back LEDs.  https://www.youtube.com/watch?v=f5ycXKbjQA8 has a demonstration of what this looks like.

I had a fun time playing with (very basic) PDP-11 assembly.  https://en.wikipedia.org/wiki/PDP-11_architecture , https://pdos.csail.mit.edu/6.828/2005/readings/pdp11-40.pdf , and https://programmer209.wordpress.com/2011/08/14/pdp-11-assembly-language-simulator/ were all great resources.

If you want to try this, replace the existing idle() in /usr/src/sys/pdp/mch_xxx.s with the following code, recompile a kernel, and reboot.

/*
 * idle()
 *
 * Sit and wait for something to happen ...
 */

/*
 * If you have a console display it's amusing to see a slowly rotating
 * sequence of lights in the display.  If the system is very active the display
 * will appear blurred.
 */
INT(LOCAL, rdisply, 037)        / idle pattern
INT(LOCAL, wcount, 8)           / rotate rdisply every wcount calls
INT(LOCAL, rotleft, 1)          / initially rotate to the left

ENTRY(idle)
        mov     PS,-(sp)        / save current SPL, indicate that no
        mov     $1,_noproc      /   process is running
        dec     wcount          / if (--wcount <= 0) {
        bgt     update  
        mov     $8,wcount       /   wcount = 8
        tst     rotleft         /   if (rotleft == 1) {
        beq     right
left:
        rol     rdisply         /     rdisply <<= 1
        bpl     update          /     if not shifted out, update
        mov     $0,rotleft      /     switch to rotating to the right
        mov     $16,wcount      /     do a slight pause before switching
        br      update          /   } else {
right:
        ror     rdisply         /     rdisply >>= 1
        bit     $1,rdisply      /     if not shifted fully right after the next loop, update
        beq     update          /
        mov     $1,rotleft      /     switch to rotating to the left
        mov     $16,wcount      /   } do a slight pause before switching
update:                         / }
        mov     rdisply,r0      / wait displays contents of r0
        SPLLOW                  / set SPL low so we can be interrupted
        wait                    / wait for something to happen
        mov     (sp)+,PS        / restore previous SPL
        rts     pc              /   and return

[Chase Covello]

Thanks, I tried this on my system and I like the pattern a lot better!

[Charley Jones]

Ooh, pretty!

Sent from my iPhone Xs!

On Oct 8, 2019, at 11:20 AM, Chase Covello <cha...@gmail.com> wrote:



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[Jeff Thieleke]

I was hoping/expecting that PDP-11 assembly experts would rip this code to pieces.  I still don't understand 1/10th of idled....  But I think I made it better by 4 bytes by using NEG instead of explicitly moving a 1 or 0 into `rotleft`








/*
 * idle()
 *
 * Sit and wait for something to happen ...
 */

/*
 * If you have a console display it's amusing to see a slowly rotating
 * sequence of lights in the display.  If the system is very active the display
 * will appear blurred.
 */
INT(LOCAL, rdisply, 037)        / idle pattern
INT(LOCAL, wcount, 8)           / rotate rdisply every wcount calls
INT(LOCAL, rotleft, 1)          / initially rotate to the left

ENTRY(idle)
        mov     PS,-(sp)        / save current SPL, indicate that no
        mov     $1,_noproc      /   process is running
        dec     wcount          / if (--wcount <= 0) {
        bgt     update  
        mov     $8,wcount       /   wcount = 8
        tst     rotleft         /   if (rotleft == 1) {

        bmi     right
left:

        rol     rdisply         /     rdisply <<= 1
        bpl     update          /     if not shifted out,

update
        neg     rotleft         /     switch to rotating to the right
        mov     $16,wcount      /     do a slight pause before switching
        br      update          /   } else {

right:
        ror     rdisply         /     rdisply >>= 1
        bit     $1,rdisply      /     if not shifted fully right after the next loop, update
        beq     update          /

        neg     rotleft         /     switch to rotating to the left
        mov     $16,wcount      /   } do a slight pause before switching
update:                         / }
        mov     rdisply,r0      / wait displays contents of r0
        SPLLOW                  / set SPL low so we can be interrupted
        wait                    / wait for something to happen
        mov     (sp)+,PS        / restore previous SPL
        rts     pc              /   and return

We could probably #ifdef this into the 2.11BSD kernel build and if this could end up as patch 452, I'd die a happy man...