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

The LowRes NX console performs operations in the order shown here:

() | ^ | - | */\ | MOD | +- | <>=<=> | NOT | AND | ORXOR | =

Global is overridden by (local) variables of the same name in subprogram arguments.

Graphics, Relation among Terms

The manual includes the following text, but I've sorted it below a bit differently to emphasize the relations between the terms.

• All graphics in LowRes NX are based on characters. A character is an 8x8-pixel image.

• Sprites are independent objects, consisting of references to characters, which can be freely moved on the screen.

• A background is a map of 32x32 characters, which is used for cell (i.e., text and tile) based maps or images. Each cell has the information of which character it references.

• The display is composed of 3 layers, which are from back to front: Background 1, Background 0, and Sprites.

Graphics, Layers

The manual's discussion of layers is incorrect (as compared to how the console behaves). The manual's text is corrected with the additions and deletions marked here:

The display is composed of 3 layers, which are from back to front:

• Background 1 (BG 1)
• Background 0 (BG 0)
• Sprites (from back sprite 63 to front sprite 0)

Each sprite and background cell has an attribute called "priority". By setting it, the cell or sprite will appear in front of the other 2 layers. on a higher display layer. Actually there are 6 layers, from back to front:

• Background 1 (BG 1) - prio 0
• Background 0 (BG 0) - prio 0
• Sprites - prio 0
• Background 1 (BG 1) - prio 1
• Background 0 (BG 0) - prio 1
• Sprites - prio 1

Graphics, Inspiring Console Tutorials

These videos explain the graphics for the historic console that inspired this fantasy console:

• How Graphics worked on the Nintendo Game Boy by Modern Vintage Gamer
• Super Mario Land 2 - Memory Exploration by Retro Game Mechanics Explained

Typecasting Float to Int

• These operations each round negative floats towards negative infinity:
  • INT (as stated in the manual)
  • array indices
• These operations each round negative floats towards zero:
  • \
  • MOD
  • AND
  • OR
  • XOR
  • x and y within SPRITE n, x, y,[c]

Bitwise Operations

• IF evaluates float=0 false, float<>0 true (does not typecast to int before operation).
• NOT can yield a negative value, and true evaluations return -1 (for example, X=-1 after evaluating X=1=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 32-bit float variables (not 32 bit int variables), thus so-called integers are limited to 24 significant bits (i.e., avoid PEEKL, POKEL).
• SPRITE VIEW ON/OFF shows or hides the complete sprite layer without changing any of the sprites' settings (as stated in the manual). And, while hidden, both SPRITE instructions still work (the screen just doesn't show any sprites).
• SPRITE OFF [n] and SPRITE OFF a TO b hide sprite(s) (as stated in the manual). And, sprite settings are preserved. And, to show sprites again, SPRITE n,x,y,[c] must be used (rather than SPRITE n,[x],[y],[c] or SPRITE n [attr ...] or SPRITE VIEW ON as the manual might imply).
• Variables are not explicitly declared, but they need to be initialized with a value before you can read from them (as stated in the manual.) And, the equal symbol is "when" the assignment occurs, e.g., X=X+1 is a valid initialization. (This is useful inside nested loops with complex program flow.) (Note the console's developer says here the behavior isn't intentional.)
• For the COPY instruction, the manual says: The source and the destination areas may overlap. The console copies the source area to a temporary, inaccessible buffer, then copies that buffer to the destination.

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).
• RESTORE [label] is 1 cc (including the label).
• INSTR searching doesn't count towards clock cycles.
• Check out Cycle counter by Rilden to test your own instructions!

CC Measurements

For-next loops (FOR X=LO TO HI STEP S) over n iterations count as:

( 2 + CC(HI) + CC(S) ) * n + ( CC(X=LO) + CC(HI) + CC(S) )

where CC(EXPRESSION) notation means clock-cycles of expression.

CC     DESCRIPTION
3*N+4    FOR X=LO   TO HI
3*N+6    FOR X=LO+1 TO HI
4*N+5    FOR X=LO   TO HI   STEP 1
6*N+7    FOR X=LO   TO HI   STEP 1+0
5*N+6    FOR X=LO   TO HI-1
6*N+7    FOR X=LO   TO HI-1 STEP 1

TRUE  FALSE  DESCRIPTION (INCLUDES ALL ASSEMBLY JUMPS, EXCLUDES ? EVALUATION)
  1     1      IF ? THEN ...
  2     1      IF ? THEN ...                    ELSE ...
  2   T2 F2    IF ? THEN ... ELSE IF ? THEN ...
  2   T3 F2    IF ? THEN ... ELSE IF ? THEN ... ELSE ...
  2     1      IF ? THEN ...                             END IF
  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

Misc. Stuff

• PRINT includes a WAIT 2 instruction after scrolling and printing the first character on the bottom line
• Single-line IF A THEN ... ELSE IF B THEN ... ELSE IF C THEN ... ELSE ... will execute the right-most ELSE ... when any of A, B, or C are false.
• ASC( A$ ) returns a signed value (rather than unsigned)
• Numbers are coded in single-precision floating point format.
• Arrays are limited to four dimensions and 2^15 elements total ('total' means multiplicative product of all dimensions)
• Strings have no effective length limit (besides clock cycles)

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 :)