A little Propeller Pasm fun tied up in yet another puzzle - novice
Beau Schwabe
Posts: 6,568
There is a fair amount of FUN-damental Pasm going on here, and I leave it to you to exercise your mind to realize it for yourself.
{{
Uses Propeller Demo Board LEDs
}}
PUB PAsm
cognew(@Entry,0)
DAT
Entry
mov dira, LED_mask
or MainLoop, temp1
or NoEnd, temp1
or Jump1, temp1
andn MainLoop, temp2
''#################################################################################################
''#################################################################################################
'' This the only line that you need to change. What happens when you comment it and why?
nop
''#################################################################################################
''#################################################################################################
MainLoop jmp AlternativeJump
mov outa, LED_Pattern1
Jump1 jmp NoEnd
AlternativeJump
mov outa, LED_Pattern2
NoEnd jmp NoEnd
LED_mask long %11111111<<$10
LED_Pattern1 long %11100111<<$10
LED_Pattern2 long %10110011<<$10
temp1 long 1<<$16
temp2 long $F<<$12
Comments
Here is a way to get rid of the 'nop' ... commentary now applies to the 'andn' instruction.
I should have titled this "jumping without using #'s" :smilewinkgrin:
{{ Uses Propeller Demo Board LEDs }} PUB PAsm cognew(@Entry,0) DAT Entry mov dira, LED_mask or MainLoop, temp1 andn MainLoop, temp2 or NoEnd, temp1 or Jump1, temp1 MainLoop jmp AlternativeJump mov outa, LED_Pattern1 Jump1 jmp NoEnd AlternativeJump mov outa, LED_Pattern2 NoEnd jmp NoEnd LED_mask long %11111111<<$10 LED_Pattern1 long %11100111<<$10 LED_Pattern2 long %10110011<<$10 temp1 long 1<<$16 temp2 long $F<<$12Another vote for editable thread titles!
Can you please comment this code line by line?
For me, this will "simply" crash....
DAT Entry mov dira, LED_mask or MainLoop, temp1 or NoEnd, temp1 or Jump1, temp1 andn MainLoop, temp2MainLoop, NoEnd and Jump1 contain the address of those locations in the code. If you modify them jump on jmp NoEnd, you never know where you will land since NoEnd can be different each time you adjust you code?JM
So what do you think happens when or MainLoop, temp1 is executed?
Based on the top posting, MainLoop is a label/name for cog register $006 holding the value $5C3C0009.
01011100001111000000000000001001 or
00000000010000000000000000000000
01011100011111000000000000001001 = $5C7C0009
So now, the address pointed by MainLoop is no more the same and point somewhere else.
I now have to apply the andn.
temp2=$F<<$12=1111000000000000000000
NOT temp2=0000111111111111111111
01011100011111000000000000001001 and
00000000000000111111111111111111 =
00000000000000000000000000001001 = 5
So now, instead of try to jump to cog register $006, it's trying to jump to cog register $005.
Honestly, I really don't understand that
JM
Cog registers are 32bit, so you have to include the top bits as well.
Again, MainLoop hasn't changed (it's still $006), just its content. What said content means - once program execution reaches that point - is for you to figure out
"Hi Beau,
Can you please comment this code line by line?"
DAT Entry mov dira, LED_mask or MainLoop, temp1 or NoEnd, temp1 or Jump1, temp1 andn MainLoop, temp2...I can't do that without giving too much away. However I can give a clue.
Take a look at MainLoop jmp AlternativeJump that is further down in the program.
Mainloop can be viewed as a variable name (long) with it's contents containing
the PAsm instruction.
In fact if you knew for certain what the proper source/destination fields would
be, you could replace that line with something like...
Mainloop long %0101110001111100000000sssssssss
...where 's' would be the literal address you wanted to jump.
So what does the PAsm instruction look like? (You can see a hint above)
Well looking at the 32 bit instruction format for jmp it looks like this...
-INSTR- ZCRI -CON- -DEST- -SRC- 010111 000i 1111 ddddddddd sssssssss Where: 's' is the register or the 9-bit literal 'd' in this case is not used and can be anything 'i' gets set depending on if you specify a register or 9-bit literal. 0 = register (use the contents of the register) 1 = 9-bit literal (use the register address)... notice in the code MainLoop jmp AlternativeJump that I'm specifying a
'register' and NOT a '9-bit literal' value.
Ok, so by itself, that line of code would be dangerous and jump somewhere
completely undetermined, leading to the common gotcha with using the # that
has bitten us all.
However, look further up in the code where you see or MainLoop, temp1
This line of code basically forces the 'i' to be a logical 1 and that's all
it does. None of the other 31 bits are affected.
remember...
'i' gets set depending on if you specify a register or 9-bit literal.
By forcing it to a 1, we are telling it to be a literal. This eliminates
the dangerous 'blind jump' that might otherwise be possible.
This should provide clues to what the remainder of the program is doing. If
not just ask, this is an academic puzzle and I really hope people looking at
this have that "ah ha!" moment. :smilewinkgrin: