BPL-BMI-BRANCHING-AND-FEW-OTHER-THINGS-FOR-NEXT-LOOP
--------------------------------------------------------------------------------------------------
TOOL: 		ML-Monitor {Commodore C128}
Language: 	Assembler 
Extra:		-
Author: 	Misel Zivanovic
Note:		(C) 26th FEB 2026  - www.lived.ch, mzretro.ch
---------------------------------------------------------------------------------------------------

We know BEQ, BNE, BCS and BCC so far and can use them efficiently or at least we believe it so! ;-)

Now there are two more branching fellows, namely BPL and BMI.
One is branching on positive result, the other one on negative. 
And they can be used for the FOR NEXT LOOPS too if it is done smart and, of course, YES WE CAN!

Let's see the branching first.

a 01300  ad 00 14 lda $1400
a 01303  10 4b    bpl $1350
a 01305  30 59    bmi $1360
a 01307  60       rts
a 01308

a 01350  a9 05    lda #$05	- green color
a 01352  8d 20 d0 sta $d020
a 01355  ee 21 d0 inc $d021	- adding next color to existing value	
a 01358  60       rts
a 01359

a 01360  a9 02    lda #$02	- red color
a 01362  8d 21 d0 sta $d021
a 01365  ee 20 d0 inc $d020	- adding next color to existing value
a 01368  60       rts
a 01369
------------------------------------
poke 5120,100:sys 4864:rem branching

As you can see, we will read the value at the location $1400, dec $5120 and branch then depending on
value. Because we are in charge, we can set the value to see how branching is behaving in under our command!

The value of 100 is branching to where? GREEN or RED?...

	- It was GREEN, of course, it is a positive number!


poke 5120,150:sys 4864:rem branching

	- It went to RED screen, but why exactly? 150 looks pretty positive too!

Actually, how do you write a negative number in assembler? Sure not like this -100, or!?
That is true, -100 won't do the job.
In 8 bit the usual suspects are the 8-bit guys and the values are from 0 to 255 = 256 values!
The first half of it is considered as a positive area, and the second part is then the negative area.
Actually it's that easy to understand what is considered a positive and what a negative number {area}.
Accordingly, branching shouldn't be a mystery anymore.

So, we got GREEN in first example because that is the first area {BPL}, branch if the result is positive.
150 is beyond the first area and is therefore a negative value. Ergo, we jumped with BMI to RED and BPL was ignored.

  0 - 127 = POSITIVE NUMBERS
128 - 255 = NEGATIVE NUMBERS 

That is clear now!
But how do we use it for the NEXT FOR LOOPS?

EXAMPLE 1:

        15 little balls

   $80> QQQQQQQQQQQQQQQ <$8e

        0123456789abcde



a 01300  a2 8e    ldx #$8e
a 01302  a9 51    lda #$51
a 01304  9d 00 04 sta $0400,x
a 01307  ca       dex
a 01308  30 fa    bmi $1304
a 0130a  60       rts
a 0130b
x
ready.
sys 4864

As you can see, it will not go below $80, dec 128, because that is a positive area!
BMI and BPL can be used for many things in order to save cycles and bytes where needed or just for fun of it!
Or even because of why not! Hehe...

In this case below, we have following scenario:

Screen start address $0400, dec 1024 + #$8e, dec 142 + dex {decrement the value} down to 128 because
that is the first negative number and if BMI sees #$7f, dec 127 {positive area}, the loop will end and exit.


EXAMPLE 2: {sys 4864 or sys 5120}

a 01300  a2 00    ldx #$00
a 01302  a9 ff    lda #$ff
a 01304  9d 00 0e sta $0e00,x
a 01307  e8       inx
a 01308  e0 40    cpx #$40
a 0130a  d0 f8    bne $1304
a 0130c  60       rts
a 0130d

this example will fill the first sprite
area with #$ff, dec 255 and it will give
us a rectangle

(as seen in the next screenshot)

a 01400  a2 40    ldx #$40
a 01402  a9 00    lda #$ff
a 01404  9d 00 0e sta $0e00,x
a 01407  ca       dex
a 01408  10 fa    bpl $1404
a 0140a  60       rts

this one will do the same, just with two
bytes less!