4. Advanced Arithmetic Operations As a refresher, here are some fundamental AVR arithmetic and logical instructions. Mnemonic Operand Description Example Notes ADD Rd, Rr Add ADD R1, R2 Rd = Rd + Rr ADC Rd, Rr Add with Carry ADC R1, R2 Rd = Rd + Rr + C (Carry flag) ADIW Rd, K Add Immediate to Word ADIW R24, 40 R[d+1]:Rd = R[d+1]:Rd + K SUB Rd, Rr Subtract SUB R16, R17 Rd = Rd - Rr SBC Rd, Rr Subtract with Carry SBC R16, R17 Rd = Rd - Rr - C SUBI Rd, K Subtract Immediate SUBI R16, 67 Rd = Rd - K SBIW Rd, K Substract Immediate from Word SBIW R24, 40 R[d+1]:Rd = R[d+1]:Rd - K SBCI Rd, K Subtract Immediate with Carry SBCI R17, 0 Rd = Rd - K - C INC Rd Increment INC R16 Rd = Rd + 1 DEC Rd Decrement DEC R16 Rd = Rd - 1 MUL Rd, Rr Multiply Unsigned MUL R16, R17 R1:R0 = Rd × Rr (16-bit result) MULS Rd, Rr Multiply Signed MULS R16, R17 R1:R0 = Rd × Rr (signed) NEG Rd Negate (Two's Complement) NEG R16 Rd = 0x00 - Rd AND Rd, Rr Logical AND AND R1, R2 Rd = Rd AND Rr ANDI Rd, K AND Immediate ANDI R16, 0x0F Rd = Rd AND K OR Rd, Rr Logical OR OR R1, R2 Rd = Rd OR Rr ORI Rd, K OR Immediate ORI R16, 0x80 Rd = Rd OR K EOR Rd, Rr Exclusive OR EOR R16, R17 Rd = Rd XOR Rr COM Rd One's Complement COM R16 Rd = 0xFF - Rd (inverts all bits) NEG Rd Two's Complement NEG R16 Rd = -Rd (Signed) CLR Rd Clear Register CLR R16 Rd = 0 SER Rd Set Register SER R16 Rd = 0xFF (R16-R31 only) Register Pair Arithmetic To perform arithmetic operations with 16 bit words (0 - 65535), we can utilize the Carry Flag to perform ripple carry arithmetic operations. 16 Bit Word Addition Suppose you want to add two big 16 bit numbers 26983 and 4882 stored in two register pairs R16:R17 and R18:R19. You can add the lower byte first then add the higher byte with a carry addition. ; DEC 26983 = HEX 0x6967 LDI R16, 0x69 ; upper byte LDI R17, 0x67 ; lower byte ; DEC 4882 = HEX 0x1312 LDI R18, 0x13 ; upper byte LDI R19, 0x12 ; lower byte ADD R17, R19 ; add lower byte ADC R16, R18 ; add upper byte + carry from lower ; R16:R17 = 31865 16 Bit Word Substraction A similar thing can be done for substracting two 16 bit numbers. ; DEC 26983 = HEX 0x6967 LDI R16, 0x69 ; upper byte LDI R17, 0x67 ; lower byte ; DEC 4882 = HEX 0x1312 LDI R18, 0x13 ; upper byte LDI R19, 0x12 ; lower byte SUB R17, R19 ; substract lower byte SBC R16, R18 ; substract upper byte - borrow (C) from lower ; R16:R17 = 22101 Immediate Arithmetic There are 3 immediate arithmetic instructions: ADIW , SBIW , and SUBI . In AVR there is no "Add with immediate" instruction. To do so, you can utilize the SUBI Rd, K instruction to substract a negative immediate number to achieve addition. Suppose you want to add 67 to 61 stored in R16. You can do the following LDI R16, 61 SUBI R16, -67 ; - -61 = + 61 ; R16 = 128 Notice how there are only immediate instructions for word addition and substraction? Recall that word immediate addressing ( ADIW & SBIW ) can only operate on 6 bit values (0 - 63). How can we perform immediate arithmetic operations on bigger numbers like 26983? Word Immediate Arithmetic To perform addition/substraction operation with immediate numbers greater than 63, we simply split the word operation into two byte immediate operations similar to the previously explained 16 bit operations. 16 Bit Immediate Addition ; DEC 4882 = HEX 0x1312 LDI R16, 0x13 ; upper byte LDI R17, 0x12 ; lower byte .EQU num, 0x5457 ; immediate constant DEC 21591 SUBI R17, lo8(-num) ; 0x12 - -0x57 = 0x12 + 0x57 ; add lower byte SBCI R16, hi8(-num) ; 0x13 - -0x54 - C = 0x13 + 0x54 + C ; add upper byte ; R16:R17 = 26473 16 Bit Immediate Substraction LDI R16, 0x67 ; yeah you get the idea atp LDI R17, 0x69 .EQU num, 0x1312 SUBI R17, lo8(num) ; this time we simply just substract it SBCI R16, hi8(num) ; no need to be negative :) ; R16:R17 = 26983 Multiplying Numbers There are multiple multiplication instructions in AVR, each supporting different data formats such as MUL Rd, Rr for unsigned numbers, MULS Rd, Rr for signed numbers, and MULSU Rd, Rr for multiplying a signed with an unsigned number. Generally, the way they behave are similar: they multiply Rd with Rr then storing a 16 bit result in R1:R0. LDI R16, 23 LDI R17, 9 MUL R16, R17 ; R1:R0 = 23 * 9 = 207 (16 bits wide)