How To

My Notes on How LowRes NX Works

nathanielbabiak 2020-11-24 12:26 (Edited)

I thought someone might find these useful, figured I'd share my notes on how LowRes NX works (and common mistakes I keep making)...

Order of Operations

^ | - | * / \ | MOD | + - | = <> | NOT | AND | OR XOR

Bitwise Operations

• Int div (\), integer modulus (MOD), boolean operators (NOT, AND, OR, XOR), and array indexes each typecast to int before operating.
• Conditionals<>0 eval true, =0 eval false, applies to both int and float.
• Float coded as 4 byte single-precision w/out div-by-zero or inf.
• NOT can yield a negative value, and TRUE is a negative value (-1).
• Negative ints don't shift rightwards (X\2^N) predictably. They're signed!
• Negative ints shift leftwards (X*2^N) predictably.
• Positive ints shift leftwards (X\2^N) or rightwards (X*2^N) predictably.

System Stuff

• CHR$(10) prints/traces a new line, VAL("0X"+NUMERALS$) converts hex.
• Verify interactions between ROM(0) and ROM(2), but not yet...
• LowRes NX treats all numeric variables as float (never int), thus so-called integers are limited to 24 sequential bits (i.e., maybe avoid PEEKL, POKEL).

Clock Cycles (cc) and Tokens

• Parenthesis don't count towards clock cycles, even when changing order of operations.
• Parenthesis do count towards tokens, as does each line (even a commented line).
• All constants are positive (negation and a constant would be 2 cc).
• Memory modification counts number of bytes modified in addition to command and arguments.
• POKEW doesn't count as memory modification, just one cc.
• CALL subprogram is 1 cc.
• CALL subprogram(0) is 2 cc (it's just a regular argument).
• SUB SUBPROGRAM is 0 cc, even with arguments.
• END SUB is 1 cc (counts towards raster interrupt subprogram's cc).

CC Measurements

'True  False  Description (each count includes all assembly jumps, excludes ? evaluation.)
'  1     1    IF ? THEN ...
'  2     1    IF ? THEN ... END IF
'  2     1    IF ? THEN ... ELSE ...
'  2     2    IF ? THEN ... ELSE ... END IF
'  2   t3 f2  IF ? THEN ... ELSE IF ? THEN ... END IF
'  3   t3 f3  IF ? THEN ... ELSE IF ? THEN ... ELSE ... END IF
'Constant "FOR X = LO TO HI ... NEXT X" loops cost 4 cc initially and 3 cc per iter.
'Iter   inner cc per iter   inner cc   total cc   total outer cc
'  0            0               0          4              4
'  1            3               3         10              7
'  2            3               6         16             10
'  3            3               9         22             13
'  1            4               4         11              7
'  2            4               8         18             10
'  3            4              12         25             13
'  1            5               5         12              7
'  2            5              10         20             10
'  3            5              15         28             13
'Expr "FOR X = LO TO HI-1 ... NEXT X" loops cost 6 cc initially and 5 cc per iter.
'Iter   inner cc per iter   inner cc   total cc   total outer cc
'  0            0               0          6              6
'  1            3               3         14             11
'  2            3               6         22             16
'  3            3               9         30             21
'  1            4               4         15             11

rilden 2020-11-24 16:40

Another thing I found out:
LowRes NX waits 2 frames when PRINT causes text to scroll up. So if you print a lot of lines, the text will scroll at 30 frames per second.

was8bit 2020-11-24 17:13

Unless you use PRINT TEXT$;

Then it only scrolls when it reaches the end of the screen

was8bit 2020-11-24 17:15

Plus, if you already use WAIT VBL in a DO LOOP, that will also add to the timing of things..

Timo 2020-11-26 18:39

"Boolean (AND/OR/NOT) operators treat any float as 0."

What do you mean? I believe both values are casted to integer and then the operator is applied. All logical operators are also binary operators by the way. That's why "true" is -1 (all bits set).

"VAL("0X"+NUMERALS$) converts hex."

This one is not on purpose. I just used the C function internally. I didn't know it does this :O

"Figure out display order of PRINT vs BG 0/1 and PRIO 0/1, but not yet..."

PRINT uses the current WINDOW. And the window command lets you set the BG.

nathanielbabiak 2020-11-27 00:23

Yep - my mistake. I only ever tested floats between 0 and 0.9999, never 1.0 or higher.

qwaffe 2020-11-27 05:28 (Edited)

this post can really benefit from some markdown formatting. i thought they were random numbers at first.

nathanielbabiak 2020-11-27 16:03 (Edited)

How do I add monospace? I tried double back-quotes (``) but it didn't work :-(

qwaffe 2020-11-28 02:36 (Edited)

put the text inside the back-quotes like normal quotes on single lines. (can be in-line.)

but code blocks are also possible. this can be quite helpful (:

nathanielbabiak 2020-11-28 03:34 (Edited)

Timo, I had to look up that emoji! Can you keep the hex conversion feature and the TRACE CHR$(10) feature? It'd be really nice to make them official...

was8bit 2020-11-28 05:28

I kinda would like to see VAL("%10") =2 and not 0... but its prolly not worth the effort to put it in...

Timo 2020-11-28 08:50 (Edited)

CHR$(10) is as expected so it’s official.

VAL(“0xFF”) doesn’t really make sense in the BASIC world as 0x is not the way here to indicate hex (it’s #).
I will probably not remove it, but I won’t add a binary version.
The only reason I see for it would be doing binary operations using strings instead of bit operators (AND OR XOR), but this is bad for performance and I won’t do this favor for was8bit ;)

was8bit 2020-11-28 09:12

oh my poor bit-brain 0o0 ;)

qwaffe 2020-11-28 11:19

isn't the way to indicate hex in nx "$" and not "#"?

was8bit 2020-11-28 12:25 (Edited)

Corerect, # and $ are side by side on the keyboard, i am sure that was just a typo... just for the record....

Hexadecimal and binary notation can be used for number values:
$FF02
%11001011

Timo 2020-11-28 12:59

Sorry, yes it's $. Wasn't a typo, I really don't know how my language works :O

was8bit 2020-11-28 15:08

You are busy with real life, its ok :)