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CCS has impressive bit handling.

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allenhuffman

Joined: 17 Jun 2019
Posts: 540
Location: Des Moines, Iowa, USA

CCS has impressive bit handling.

Posted: Tue Feb 25, 2020 1:35 pm

I read about int1, but never used it. The compiler will use one byte for up to 8 of those:

Code:

int1 bit1;
int1 bit2;
int1 bit3;
int1 bit4;
int1 bit5;
int1 bit6;
int1 bit7;

bit0 = 1;
bit1 = 0;
bit2 = 1;
bit3 = 0;
bit4 = 1;
bit5 = 0;
bit6 = 1;
bit7 = 0;

Those turn into nice atomic assembly calls:

Code:

.................... bit0 = 1;
00228: BSET.B 800.0
.................... bit1 = 0;
0022A: BCLR.B 800.1
.................... bit2 = 1;
0022C: BSET.B 800.2
.................... bit3 = 0;
0022E: BCLR.B 800.3
.................... bit4 = 1;
00230: BSET.B 800.4
.................... bit5 = 0;
00232: BCLR.B 800.5
.................... bit6 = 1;
00234: BSET.B 800.6
.................... bit7 = 0;
00236: BCLR.B 800.7

So I wondered how different it was to doing it manually.

Just SETTING a bit like this is larger:

Code:

.................... byte = (1 << 4);
00238: MOV.B #10,W0L
0023A: MOV.B W0L,801

But ORing a bit generates the same nice code without needing to use non-standard int1:

Code:

.................... byte = byte | (1 << 4);
0023C: BSET.B 801.4

Clearing a bit also generates one atomic instruction:

Code:

.................... byte = byte & ~(1 << 4);
0023E: BCLR.B 801.4

Kudos to CCS. It's nice seeing int1 available, but nicer seeing the compiler is smart enough to do bit set/clear efficiently if you are manually doing it.

I created Arduino-compatible bit macros and worried they might generate bulky code, but I never checked until now:

Code:

.................... byte = bitSet(byte, 4);
0023C: BSET.B 801.4

.................... byte = bitClear(byte, 4);
00242: BCLR.B 801.4

_________________
Allen C. Huffman, Sub-Etha Software (est. 1990) http://www.subethasoftware.com
Embedded C, Arduino, MSP430, ESP8266/32, BASIC Stamp and PIC24 programmer.
http://www.whywouldyouwanttodothat.com ?

temtronic

Joined: 01 Jul 2010
Posts: 9204
Location: Greensville,Ontario


Posted: Tue Feb 25, 2020 5:19 pm

Really has nothing to do with CCS, thank MICROCHIP ! They're the ones that embedded bit instructions in the PIC !!

allenhuffman

Joined: 17 Jun 2019
Posts: 540
Location: Des Moines, Iowa, USA

Posted: Wed Feb 26, 2020 1:32 pm

temtronic wrote:

Really has nothing to do with CCS, thank MICROCHIP ! They're the ones that embedded bit instructions in the PIC !!

I am glad to give credit to CCS for actually taking advantage of it Smile

I've seem some terrible code generation over the years, so I'm always pleased when I see a compiler doing something smart.

I noticed that bit set/clear when looking at the I2C assembly. Nice.
_________________
Allen C. Huffman, Sub-Etha Software (est. 1990) http://www.subethasoftware.com
Embedded C, Arduino, MSP430, ESP8266/32, BASIC Stamp and PIC24 programmer.
http://www.whywouldyouwanttodothat.com ?

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