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digitalmars.D.learn - signed nibble

reply Michelle Long <HappyDance321 gmail.com> writes:
Is there any direct way to convert a signed nibble in to a signed 
byte with the same absolute value? Obviously I can do some bit 
comparisons but just curious if there is a very quick way.
Jan 07 2019
next sibling parent reply Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Monday, 7 January 2019 at 17:23:19 UTC, Michelle Long wrote:

 Is there any direct way to convert a signed nibble in to a 
 signed byte with the same absolute value? Obviously I can do 
 some bit comparisons but just curious if there is a very quick 
 way.


byte b = nibble | ((nibble & 0x40)?0xF0:0);
Jan 07 2019
next sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Monday, 7 January 2019 at 18:42:13 UTC, Patrick Schluter wrote:

 byte b = nibble | ((nibble & 0x40)?0xF0:0);


don't you mean & 0x80 ?
Jan 07 2019
parent Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Monday, 7 January 2019 at 18:47:04 UTC, Adam D. Ruppe wrote:

 On Monday, 7 January 2019 at 18:42:13 UTC, Patrick Schluter 
 wrote:

 byte b = nibble | ((nibble & 0x40)?0xF0:0);


 don't you mean & 0x80 ?


He asked for signed nybble. So mine is wrong and yours also :-)

It's obviously 0x08 for the highest bit of the low nybble.

byte b = nibble | ((nibble & 0x08)?0xF0:0);
Jan 07 2019
prev sibling parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Mon, Jan 07, 2019 at 06:42:13PM +0000, Patrick Schluter via
Digitalmars-d-learn wrote:

 On Monday, 7 January 2019 at 17:23:19 UTC, Michelle Long wrote:

 Is there any direct way to convert a signed nibble in to a signed
 byte with the same absolute value? Obviously I can do some bit
 comparisons but just curious if there is a very quick way.


 
 byte b = nibble | ((nibble & 0x40)?0xF0:0);


This is equivalent to doing a bit comparison (implied by the ?
operator).  You can do it without a branch:

	cast(byte)(nibble << 4) >> 4

will use the natural sign extension of a (signed) byte to "stretch" the
upper bit.  It just takes 2-3 CPU instructions.


T

-- 
Written on the window of a clothing store: No shirt, no shoes, no service.
Jan 07 2019
parent reply Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Monday, 7 January 2019 at 18:56:17 UTC, H. S. Teoh wrote:

 On Mon, Jan 07, 2019 at 06:42:13PM +0000, Patrick Schluter via 
 Digitalmars-d-learn wrote:

 On Monday, 7 January 2019 at 17:23:19 UTC, Michelle Long wrote:

 Is there any direct way to convert a signed nibble in to a 
 signed byte with the same absolute value? Obviously I can do 
 some bit comparisons but just curious if there is a very 
 quick way.


 
 byte b = nibble | ((nibble & 0x40)?0xF0:0);


 This is equivalent to doing a bit comparison (implied by the ? 
 operator).  You can do it without a branch:

 	cast(byte)(nibble << 4) >> 4

 will use the natural sign extension of a (signed) byte to 
 "stretch" the upper bit.  It just takes 2-3 CPU instructions.


Yeah, my bit-fiddle-fu goes back to pre-barrel-shifter days. Up 
to 32 bit processors, shifting was more expensive than branching.
Jan 07 2019
parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Mon, Jan 07, 2019 at 08:06:17PM +0000, Patrick Schluter via
Digitalmars-d-learn wrote:

 On Monday, 7 January 2019 at 18:56:17 UTC, H. S. Teoh wrote:

 On Mon, Jan 07, 2019 at 06:42:13PM +0000, Patrick Schluter via
 Digitalmars-d-learn wrote:




[...]

 byte b = nibble | ((nibble & 0x40)?0xF0:0);


 
 This is equivalent to doing a bit comparison (implied by the ?
 operator).  You can do it without a branch:
 
 	cast(byte)(nibble << 4) >> 4
 
 will use the natural sign extension of a (signed) byte to "stretch"
 the upper bit.  It just takes 2-3 CPU instructions.
 


 
 Yeah, my bit-fiddle-fu goes back to pre-barrel-shifter days. Up to 32
 bit processors, shifting was more expensive than branching.


Really?  Haha, never knew that, even though I date all the way back to
writing assembly on 8-bit processors. :-D


T

-- 
One reason that few people are aware there are programs running the internet is
that they never crash in any significant way: the free software underlying the
internet is reliable to the point of invisibility. -- Glyn Moody, from the
article "Giving it all away"
Jan 07 2019
parent reply Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Monday, 7 January 2019 at 20:28:21 UTC, H. S. Teoh wrote:

 On Mon, Jan 07, 2019 at 08:06:17PM +0000, Patrick Schluter via 
 Digitalmars-d-learn wrote:

 On Monday, 7 January 2019 at 18:56:17 UTC, H. S. Teoh wrote:

 On Mon, Jan 07, 2019 at 06:42:13PM +0000, Patrick Schluter 
 via Digitalmars-d-learn wrote:




 [...]

 byte b = nibble | ((nibble & 0x40)?0xF0:0);


 
 This is equivalent to doing a bit comparison (implied by the 
 ? operator).  You can do it without a branch:
 
 	cast(byte)(nibble << 4) >> 4
 
 will use the natural sign extension of a (signed) byte to 
 "stretch" the upper bit.  It just takes 2-3 CPU instructions.
 


 
 Yeah, my bit-fiddle-fu goes back to pre-barrel-shifter days. 
 Up to 32 bit processors, shifting was more expensive than 
 branching.


 Really?  Haha, never knew that, even though I date all the way 
 back to writing assembly on 8-bit processors. :-D


Most of my career was programming for 80186. Shifting by one was 
2 cycles in register and 15 in memory. Shifting by 4, 9 cycles 
for regs/21 for mem. And 80186 was a fast shifter compared to 
8088/86 or 68000 (8+2n cycles).
Jan 07 2019
parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Mon, Jan 07, 2019 at 08:41:32PM +0000, Patrick Schluter via
Digitalmars-d-learn wrote:

 On Monday, 7 January 2019 at 20:28:21 UTC, H. S. Teoh wrote:

 On Mon, Jan 07, 2019 at 08:06:17PM +0000, Patrick Schluter via
 Digitalmars-d-learn wrote:




[...]

 Up to 32 bit processors, shifting was more expensive than
 branching.


 
 Really?  Haha, never knew that, even though I date all the way back
 to writing assembly on 8-bit processors. :-D
 


 Most of my career was programming for 80186. Shifting by one was 2
 cycles in register and 15 in memory. Shifting by 4, 9 cycles for
 regs/21 for mem. And 80186 was a fast shifter compared to 8088/86 or
 68000 (8+2n cycles).


I used to hack 6502 assembly code.  During the PC revolution I wrote an
entire application in 8088 assembly.  Used to know many of the opcodes
and cycle counts by heart like you do, but it's all but a faint memory
now.


T

-- 
Right now I'm having amnesia and deja vu at the same time. I think I've
forgotten this before.
Jan 07 2019
parent reply Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Monday, 7 January 2019 at 21:46:21 UTC, H. S. Teoh wrote:

 On Mon, Jan 07, 2019 at 08:41:32PM +0000, Patrick Schluter via 
 Digitalmars-d-learn wrote:

 On Monday, 7 January 2019 at 20:28:21 UTC, H. S. Teoh wrote:

 On Mon, Jan 07, 2019 at 08:06:17PM +0000, Patrick Schluter 
 via Digitalmars-d-learn wrote:




 [...]

 Up to 32 bit processors, shifting was more expensive than 
 branching.


 
 Really?  Haha, never knew that, even though I date all the 
 way back to writing assembly on 8-bit processors. :-D
 


 Most of my career was programming for 80186. Shifting by one 
 was 2 cycles in register and 15 in memory. Shifting by 4, 9 
 cycles for regs/21 for mem. And 80186 was a fast shifter 
 compared to 8088/86 or 68000 (8+2n cycles).


 I used to hack 6502 assembly code.


Yeah, that's also what I started with, on the Apple II in the 
early 80s. I was quite surprized that my 6502 knowledge came in 
very handy when we worked on dial-in modems in the late 90s as 
the Rockwell modems all used 6502 derived micro-controllers for 
them.


 During the PC revolution I wrote an entire application in 8088 
 assembly.  Used to know many of the opcodes and cycle counts by 
 heart like you do, but it's all but a faint memory now.


I had to lookup the exact cycle counts ;-) . I remember the 
relative costs, more or less, but not the details anymore.
Jan 08 2019
parent reply Ola Fosheim =?UTF-8?B?R3LDuHN0YWQ=?= <ola.fosheim.grostad gmail.com> writes:
On Tuesday, 8 January 2019 at 09:30:14 UTC, Patrick Schluter 
wrote:

 During the PC revolution I wrote an entire application in 8088 
 assembly.  Used to know many of the opcodes and cycle counts 
 by heart like you do, but it's all but a faint memory now.


 I had to lookup the exact cycle counts ;-) . I remember the 
 relative costs, more or less, but not the details anymore.


Heh, I remember they had a friday-night trivia contest at the 
mid-90s students pub (for natural sciences) where one of the 
questions was the opcode for 6502 LDA (or was it NOP?), and I 
believe I got it right. The opcode for NOP is burned into my 
memory as $EA was used for erasing code during debugging in a 
monitor. And it was also the letters for the big game company 
Electronic Arts...

The cycle counts for 6502 are pretty easy though as they tend to 
be related to the addressing mode and most of them are in the 
range 1-5... No instruction for multiplication or division... Oh 
the fun...
Jan 08 2019
parent reply Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Tuesday, 8 January 2019 at 10:32:25 UTC, Ola Fosheim Grøstad 
wrote:

 On Tuesday, 8 January 2019 at 09:30:14 UTC, Patrick Schluter 
 wrote:

 [...]


 Heh, I remember they had a friday-night trivia contest at the 
 mid-90s students pub (for natural sciences) where one of the 
 questions was the opcode for 6502 LDA (or was it NOP?), and I 
 believe I got it right. The opcode for NOP is burned into my 
 memory as $EA was used for erasing code during debugging in a 
 monitor. And it was also the letters for the big game company 
 Electronic Arts...

 The cycle counts for 6502 are pretty easy though as they tend 
 to be related to the addressing mode and most of them are in 
 the range 1-5... No instruction for multiplication or 
 division... Oh the fun...


2-7 cycles ;-)
Jan 08 2019
parent Ola Fosheim =?UTF-8?B?R3LDuHN0YWQ=?= <ola.fosheim.grostad gmail.com> writes:
On Tuesday, 8 January 2019 at 10:55:59 UTC, Patrick Schluter 
wrote:

 The cycle counts for 6502 are pretty easy though as they tend 
 to be related to the addressing mode and most of them are in 
 the range 1-5... No instruction for multiplication or 
 division... Oh the fun...


 2-7 cycles ;-)


There you go, the last one in that series I touched was 6800 in 
1990.

But it kinda make sense, the instructions were not pipelined, so 
even NOP takes 2 cycles... Another world really.
Jan 08 2019
prev sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Mon, Jan 07, 2019 at 05:23:19PM +0000, Michelle Long via Digitalmars-d-learn
wrote:

 Is there any direct way to convert a signed nibble in to a signed byte
 with the same absolute value? Obviously I can do some bit comparisons
 but just curious if there is a very quick way.


Assuming you have the nibble stored in the lower bits of a ubyte:

	import std.stdio;
	byte nibSgnExt(ubyte nib)
	{
		return cast(byte)(nib << 4) >> 4;
	}
	void main() {
		writefln("%02X", nibSgnExt(0x0F));
		writefln("%02X", nibSgnExt(0x07));
	}


T

-- 
If it breaks, you get to keep both pieces. -- Software disclaimer notice
Jan 07 2019