RISC OS on ARM based CPUs - Revision history

Jin X: NEON Programmers Guide link update

2021-07-13T16:21:43Z

NEON Programmers Guide link update

Jin X at 18:26, 21 June 2021

2021-06-21T18:26:30Z

Jin X at 18:25, 21 June 2021

2021-06-21T18:25:05Z

Jin X: comma is added between ".set OS_CallASWI" and "0x6f"

2021-06-21T18:24:27Z

comma is added between ".set OS_CallASWI" and "0x6f"

Kuemmel at 20:12, 29 September 2020

2020-09-29T20:12:31Z

Kuemmel at 20:11, 29 September 2020

2020-09-29T20:11:32Z

Kuemmel: some tricks added

2020-09-29T20:09:22Z

some tricks added

Kuemmel at 08:17, 17 September 2020

2020-09-17T08:17:05Z

Kuemmel: Code Tricks added

2020-07-14T09:15:21Z

Code Tricks added

Kuemmel at 10:19, 12 July 2020

2020-07-12T10:19:41Z

@@ Line 332: / Line 332: @@
 [https://developer.arm.com/documentation/qrc0007/e Vector Floating Point Instruction Set Quick Reference Card]
-[https://static.docs.arm.com/den0018/a/DEN0018A_neon_programmers_guide_en.pdf NEON Programmer's Guide]
+[https://developer.arm.com/documentation/den0018/latest NEON Programmer's Guide]
 [http://www.keil.com/support/man/docs/armasm/armasm_dom1361289850039.htm Instruction Set Overview]

@@ Line 75: / Line 75: @@
 So for a basic intro loop in THUMB-2 this would look like
-<syntaxhighlight lang="nasm">.syntax unified
+<syntaxhighlight lang="gas">.syntax unified
 .thumb                   //assemble using thumb mode
 movs r0,#0               //reason code to set screen mode by number

@@ Line 75: / Line 75: @@
 So for a basic intro loop in THUMB-2 this would look like
-<syntaxhighlight lang="gas">.syntax unified
+<syntaxhighlight lang="nasm">.syntax unified
 .thumb                   //assemble using thumb mode
 movs r0,#0               //reason code to set screen mode by number

@@ Line 70: / Line 70: @@
 .set OS_ReadEscapeState, 0x2c
 .set OS_Exit, 0x11
-.set OS_CallASWI 0x6f
+.set OS_CallASWI, 0x6f
 </syntaxhighlight>

@@ Line 301: / Line 301: @@
 and r1,r1,#0xffff // limit to halfword (it's impossible to assemble this as 0xffff is out of range)</syntaxhighlight>
 ...but the first one assembles to 4 bytes and the later one isn't even possible to assemble (it's doable with the 4 byte instruction <code>bfc r1,16,16</code> or two shift instructions). But we are lucky and the extending instructions come in handy to do the job with just 2 byte instructions each:
-<syntaxhighlight lang="gas">uxtb r0,r0 // unsigned byte to halfword = limit to byte
+<syntaxhighlight lang="gas">uxtb r0,r0 // unsigned byte extend to word = limit to byte
 uxth r1,r1 // unsigned halfword extend to word = limit to halfword </syntaxhighlight>

@@ Line 295: / Line 295: @@
 <syntaxhighlight lang="gas">orr   r0,r0,r0,asr#31</syntaxhighlight>
 All of those simplifications can be coded in Thumb, too, but would need also two instructions each and therefore end up with the same size, so I skip that here.
 == Compression ==

@@ Line 66: / Line 66: @@
 <syntaxhighlight lang="NASM">.set OS_ScreenMode, 0x65
 .set OS_RemoveCursors, 0x36
 .set OS_ReadVduVariables, 0x31
 .set OS_ReadMonotonicTime, 0x42

← Older revision		Revision as of 09:15, 14 July 2020
Line 264:		Line 264:

	There are other ways to do sound on RISC OS, but those were not evaluated at the time of writing. Also BBC Basic has ways to create sounds by note or frequency (Link is [http://www.riscos.com/support/developers/bbcbasic/part2/sound.html here]). Check out the Sound SWI calls in detail here [https://www.riscosopen.org/wiki/documentation/show/Sound%20SWI%20Calls Sound SWI Calls]. Some further insights on the sound system can be found here [http://jcgl.orpheusweb.co.uk/temp/ROSSDocument.pdf The RISC OS sound system by j. Lesurf].		There are other ways to do sound on RISC OS, but those were not evaluated at the time of writing. Also BBC Basic has ways to create sounds by note or frequency (Link is [http://www.riscos.com/support/developers/bbcbasic/part2/sound.html here]). Check out the Sound SWI calls in detail here [https://www.riscosopen.org/wiki/documentation/show/Sound%20SWI%20Calls Sound SWI Calls]. Some further insights on the sound system can be found here [http://jcgl.orpheusweb.co.uk/temp/ROSSDocument.pdf The RISC OS sound system by j. Lesurf].
		+
		+	== Code Tricks ==
		+	=== Cheap Absolute Value ===
		+	Usually calculating ABS() would look like this in ARM (of course the cmp could be omitted if a preceding instruction triggered the flags already).
		+	<syntaxhighlight lang="gas">cmp r0,#0
		+	rsblt r0,r0,#0</syntaxhighlight>
		+	and in Thumb
		+	<syntaxhighlight lang="gas">cmp r0,#0
		+	it lt
		+	rsblt r0,r0,#0</syntaxhighlight>
		+	If your routine is okay with a small deviation (this variant will form a negative value using the formulae ABS(x) = -x - 1) this can be done by just one instruction (ARM). Hint taken from Coder's Revenge 06/96 disc mag.
		+	<syntaxhighlight lang="gas">eor r0,r0,r0,asr#31</syntaxhighlight>
		+	In Thumb this would need 2 instructions so I skip that here.
		+	=== Clamping ===
		+	Clamping towards zero or similar code cases can be optimized in the same way. Here are 3 examples also from Coder's Revenge 06/96:
		+
		+	Example 1 - All values < 0 should be 0
		+	<syntaxhighlight lang="gas">cmp r0,#0 // r0 ? 0
		+	movlt r0,#0 // r0<0 => r0=0</syntaxhighlight>
		+	can be done by
		+	<syntaxhighlight lang="gas">bic r0,r0,r0,asr#31</syntaxhighlight>
		+	Example 2 - All values >= 0 should be 0
		+	<syntaxhighlight lang="gas">cmp r0,#0 // r0 ? 0
		+	movge r0,#0 // r0>=0 => r0=0</syntaxhighlight>
		+	can be done by
		+	<syntaxhighlight lang="gas">and r0,r0,r0,asr#31</syntaxhighlight>
		+	Example 3 - All values <= -1 should be -1
		+	<syntaxhighlight lang="gas">cmn r0,#1 // r0 ? -1
		+	mvnle r0,#0 // r0<=-1 => r0=-1</syntaxhighlight>
		+	can be done by
		+	<syntaxhighlight lang="gas">orr r0,r0,r0,asr#31</syntaxhighlight>
		+	All of those simplifications can be coded in Thumb, too, but would need also two instructions each and therefore end up with the same size, so I skip that here.

	== Compression ==		== Compression ==

@@ Line 275: / Line 275: @@
 to compress your intro. As an example the intro [https://www.pouet.net/prod.php?which=85044 'blury'] was compressed from 966 to 832 Bytes.
-At the time of writing it doesn't work with Thumb code when the standard trick that is mention above is used to specify the execution address of the intro in the filing system. So for now you would have to skip that and invest 4 bytes at the start of your intro to make it compressible with Codepressor:
+Codepressor also supports Thumb executables, both, if the thumb trigger is set within the code or as shown above set within the filesystem. Future versions might also have thumb decompression routines to make the files even shorter. At the time of writing the decompression routines are written in normal ARM code.
 == Resources ==