Accessing the LSB or MSB is sometimes useful in optimisations. For example, when I am computing the address to write a pixel to the graph buffer, I often do something that includes setting the 6th bit of C and loading 93h into B.

Okay, in response to sorunome, here is an update with full rectangle clipping:
Grammer.8xk

Ah, well with that code, will draw a sprite with hexadecimal data input. By that, I mean . db "3C4281818181423C" as opposed to .db 3Ch,42h,81h,81h,81h,81h,42h,3Ch

The first is what a TI-BASIC string would be like and the program that code is in is designed for TI-BASIC programmers to use.

Unless i'm missing something, the first just says "Array", maybe you pasted the wrong thing?

I am not seeing that... On mine, Hayleia and I have the same thing...

That code doesn't change the run indicator, to my knowledge... maybe FDCB128E?

Ooh, rrd and rld are some of my favorite instructions! When I was describing what it does in my pocket asm tutorial was this:

rrd/rld the two nibbles from (hl) and the last nibble of 'a' make three nibbles. rrd                
           rotates them right and rld rotates them left.

As for a way to use them, I have a few codes that make excellent use of them. This one is from my program BSPRT. It uses hexadecimal sprite data, so instead of converting the hex string to a location in RAM and then copying it to the screen, I just did both at once (here is the sprite display part):
;Inputs:
;     HL points to the buffer location to draw to
;     DE points to the sprite data

     ld bc,080Ch     ;010C08
Loop:
     call PutNibble  ;CD****
     call PutNibble  ;CD****
     ld a,b          ;78
     ld b,0          ;0600
     add hl,bc       ;09
     ld b,a          ;47
     djnz Loop       ;10F3
     ret             ;C9
PutNibble:
     ld a,(de)       ;1A
     add $C0         ;C6C0
     jr nc,$+4       ;3002
       sub 7         ;D607
     rld             ;ED6F
     inc de          ;13
     ret             ;C9
This is my favorite one, though. When I made a graph shifting routine, I made these to shift 4 pixels very quickly:
ShiftLeft4:
     ld hl,GraphBuf+767
     ld c,64
       xor a
       ld b,12
         rld
         dec hl
         djnz $-3
       dec c
       jr nz,$-9
       ret
ShiftRight4:
     ld hl,GraphBuf
     ld c,64
       xor a
       ld b,12
         rrd
         inc hl
         djnz $-3
       dec c
       jr nz,$-9
       ret
I feel pretty proud of these I think the first can be optimised more, but it is still nice, I think

I think I've only ever uploaded half of my programs to the internet, tops For example, I have a grayscale tilemapper and tile editor written for Grammer, I have a super fast snake game that I was working on last night, I was trying to make some link routines that could send commands via the link port (like a command to obtain the other calcs keypress and graph screen) and that has not worked for me xD I made a platformer with gravity physics and jumping (not much, but still nice), and I've been working on making a program editor for Grammer. Blegh, this works for me, though. It means I finish a lot of programs around the same time, then I have a lull, then a lot of programs, and it keeps going. And I am still working on BatLib, occasionally I am actually rewriting it, so that is fun

Thanks! And yes, my avatar is made in Grammer code. It was actually made a while before I made a command to turn on pixels in a region to particles.