TXA-DO-IT
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TOOL: 		Built-In ML-Monitor {Commodore C128}
Language: 	Assembler 
Extra:		none
Author: 	Misel Zivanovic
Note:		(C) 13th DEC 2025 - www.lived.ch, mzretro.ch
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The ways to do it and the impact of it.

These assembler lines are filling first 5 rows with a little ball.
And the next 5 rows then with characters and graphics symbols C128 is offering.

As you can see, to switch between this two methods not much was necessary.
One TXA did the job for us. {Transfer X-register to Accumulator}

What is the difference between part one and part two?

1st part is taking the fill value directly from accumulator.
which is a static value of #$51 or dec 81. And that is the value of the little ball.

2nd part is using also the accumulator to fill the rows, however it's not static any
more because we are transferring the values from the X-register into accumulator.
The X-register values are incremented by one {INX} we are getting different
values which makes it possible to display different characters/symbols. 

d 1300 131a
. 01300  a2 00    ldx #$00
. 01302  a9 51    lda #$51
. 01304  9d 00 04 sta $0400,x
. 01307  e8       inx
. 01308  e0 c8    cpx #$c8
. 0130a  d0 f8    bne $1304
. 0130c  ea       nop
. 0130d  ea       nop
. 0130e  a2 00    ldx #$00
. 01310  8a       txa
. 01311  9d c8 04 sta $04c8,x
. 01314  e8       inx
. 01315  e0 c8    cpx #$c8
. 01317  d0 f7    bne $1310
. 01319  60       rts
. 0131a  ff       ???

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ALL EXPLAINED:

d 1300 131a
a 01300  a2 00    ldx #$00	Set X-register to 0
a 01302  a9 51    lda #$51	load accumulator with #$51 {dec 81} which is a little ball
a 01304  9d 00 04 sta $0400,x	start from the screen address $0400, dec 1024 {left upper corner}
a 01307  e8       inx		increment X-register
a 01308  e0 c8    cpx #$c8	Is it #$c8 or dec 200 already?
a 0130a  d0 f8    bne $1304	Not #$c8 then REPEAT. If yes then fall through here to continue!
a 0130c  ea       nop		NO Operation
a 0130d  ea       nop		{I put this just to visually separate the code
a 0130e  a2 00    ldx #$00	2-Part, Set X-register to 0
a 01310  8a       txa		TRANSFER X-register into accumulator
a 01311  9d c8 04 sta $04c8,x	Display from screen address $04c8
a 01314  e8       inx		increment X-register
a 01315  e0 c8    cpx #$c8	#$c8 reached?
a 01317  d0 f7    bne $1310	No? Repeat
a 01319  60       rts		Return To Subroutine or BASIC if called from BASIC {SYS 4864}
a 0131a  ff       ???


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MACHINE-LANGUAGE-MONITOR CODE LISTING:

{If you paste this code into ML-Monitor of C128, then you can avoid manual corrections which}
{are practically mandatory when you paste the assembled code from above}
{This below is exactly the same code as above!}
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m 1300 131a

>01300 a2 00 a9 51 9d 00 04 e8:B.Iq...H
>01308 e0 c8 d0 f8 ea ea a2 00:.HPXJJB.
>01310 8a 9d c8 04 e8 e0 c8 d0:..H.H.HP
>01318 f7 60 ff ff ff ff 00 00:W.......

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OPTIMIZATION

Can this code be optimized!?
We wasted 26 bytes {incl. NOP's} to achieve the seen results! Not much actually...

Well, it can and here it is.

These 17 bytes are doing the same as previously 26 {or 24 if we exclude the NOP's)
That is 7 bytes less!

d 1300
a 01300  a2 00    ldx #$00
a 01302  a9 51    lda #$51
a 01304  9d 00 04 sta $0400,x
a 01307  8a       txa
a 01308  9d c8 04 sta $04c8,x
a 0130b  e8       inx
a 0130c  e0 c8    cpx #$c8
a 0130e  d0 f2    bne $1302
a 01310  60       rts
a 01311  00       brk