I have code producing strange results. The application uses an 8-bit
counter and overflow counts to measure distance with an ultrasonic
transponder. The overflow and count look Ok. Fitting an equation to
experimental data, I get
dist = 0.013657 * cnts + -3.996578.
Converting the multiplication, I have
distance = (overflow*256 + count)*895/2^16 - 4
Where 895 is 0.013657 * 65536
I cannot find what I am doing wrong. Must be something simple.
I want to multiply a 16-bit value by a 16-bit value and keep the high
order 16 bits, using only the low 8 bits of the result.
extern volatile uint8_t range_local_ovfl;
extern volatile uint8_t range_local;
extern volatile uint8_t range_count;
if (range_new == TRUE)
{
{
uint32_t distance = range_local_ovfl<<8;
distance += (uint32_t)range_count;
distance = =895*distance;
distance = distance >> 16;
range_local = (uint8_t)(distance - 4);
}
...
}
895*distance generates a call to __mulsi3, multiplying r25:22 by r21:18
and returning the result in r25:22.
However, I cannot find where r21:18 are set in the code. Looking at the
output of avr-gcc -S, I cannot find any of r18:21.
avr-gcc -S -mmcu=atmega32 -Wall -Wmissing-prototypes -Os
-fno-strict-aliasing -I../include -I../../../avrx2.6 -I.
-I../../../../asus-avr-1.0/include -c ../common/range.c -o -
| grep r18
produces nothing. Same for all of r19..r21
>From avr-objdump,
1bce: 0e 94 e3 12 call 0x25c6 ; 0x25c6<AvrXStartTimer>
1bd2: 80 91 c7 02 lds r24, 0x02C7
1bd6: 81 30 cpi r24, 0x01 ; 1
1bd8: f9 f4 brne .+62 ; 0x1c18 <range+0x60>
1bda: 80 91 96 02 lds r24, 0x0296
1bde: 18 2f mov r17, r24
1be0: 00 e0 ldi r16, 0x00 ; 0
1be2: b8 01 movw r22, r16
1be4: 88 27 eor r24, r24
1be6: 77 fd sbrc r23, 7
1be8: 80 95 com r24
1bea: 98 2f mov r25, r24
1bec: e0 91 87 02 lds r30, 0x0287
1bf0: 6e 0f add r22, r30
1bf2: 71 1d adc r23, r1
1bf4: 81 1d adc r24, r1
1bf6: 91 1d adc r25, r1
1bf8: 0e 94 ec 16 call 0x2dd8 ; 0x2dd8 <__mulsi3>
1bfc: bc 01 movw r22, r24
1bfe: 88 27 eor r24, r24
1c00: 99 27 eor r25, r25
1c02: 64 50 subi r22, 0x04 ; 4
1c04: 60 93 b6 02 sts 0x02B6, r22
1c08: 80 91 87 02 lds r24, 0x0287
1c0c: 80 93 7a 02 sts 0x027A, r24
1c10: 80 91 c6 02 lds r24, 0x02C6
1c14: 80 93 b9 02 sts 0x02B9, r24
1c18: c0 9a sbi 0x18, 0 ; 24
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