It's really interesting to see that it could be done like that, but, that pseudo code you've given me would translate to a heck of a lot of MCS-48 instructions! 16-bit subtraction in particular is really expensive. Potentially not much gain there?
; INPUT: r1:r0 celsius * 10
; OUTPUT: r3:r2 fahrenheit * 10
MOV A, r0
ADD A, r0
MOV r2, A
MOV r4, A
MOV A, r1
RLC A
MOV r3, A
MOV r5, A
MOV A, r4
ADD A, r4
XCH A, r5
RLC A
MOV r4, A
MOV A, r2
ADD A, r5
XCH A, r3
ADDC A, r4
MOV r2, A
MOV A, r5
ADD A, r5
XCH A, r4
RLC A
MOV r5, A
MOV A, r3
ADD A, r4
XCH A, r2
ADDC A, r5
MOV r3, A
MOV A, r1
ADD A, r1
MOV A, r1
RRC A
MOV r5, A
MOV A, r0
RRC A
MOV r4, A
MOV A, r5
ADD A, r5
MOV A, r5
RRC A
XCH A, r4
RRC A
MOV r5, A
MOV A, r2
ADD A, r5
XCH A, r3
ADDC A, r4
MOV r2, A
MOV A, r4
ADD A, r4
MOV A, r4
RRC A
XCH A, r5
RRC A
MOV r4, A
MOV A, r3
ADD A, r4
XCH A, r2
ADDC A, r5
MOV r3, A
MOV A, r5
ADD A, r5
MOV A, r5
RRC A
XCH A, r4
RRC A
MOV r5, A
MOV A, r4
ADD A, r4
MOV A, r4
RRC A
XCH A, r5
RRC A
MOV r4, A
MOV A, r2
ADD A, r4
XCH A, r3
ADDC A, r5
ADD A, 10
MOV r2, A
ADD A, r2
MOV A, r2
RRC A
XCH A, r3
RRC A
MOV r2, A
MOV A, r3
ADD A, r3
MOV A, r3
RRC A
XCH A, r2
RRC A
MOV r3, A
MOV A, r2
ADD A, r2
MOV A, r2
RRC A
XCH A, r3
RRC A
MOV r2, A int16_t celsius2fahrenheit(int16_t c)
{
int16_t f = 0;
f += c << 1;
f += c << 2;
f += c << 3;
f += c >> 2;
f += c >> 3;
f += c >> 5;
f += 320 * 8;
f >>= 3;
return f;
}