Instruction sets of a microcontroller

DATA processing instructions :
We use data processing instructions to manipulate data within the registers. 

Types for data processing instructions – 

• Arithmetic instructions
• Logical instructions
• Multiply instructions
• Comparison instructions
• Move instructions

 Most of the data processing instructions use a barrel shifter to pre-process the data of one of their operands. Each operation updates different flags in the cpsr (To learn more about ‘CPSR’ search “CPSR in ARM”). 

Let us discuss the instructions in detail.

1. Arithmetic instructions :
The arithmetic instructions mainly implement addition and subtraction of 32bit signed and unsigned values.

                        Syntax: <instruction>{<cond>}{s} Rd, Rn, N

ADC	add with 32-bit values and carry	Rd=Rn+N+carry
ADD	add two 32-bit values	Rd=Rn+N
RSB	reverse subtract of two 32-bit values	Rd=N-Rn
RSC	reverse subtract with carry of two 32-bit values	Rd=N-Rn-!(Carry flag)
SBC	subtract with carry of two 32-bit values	Rd=Rn-N-!(Carry flag)
SUB	subtract two 322-bit values	Rd=Rn-N

N is the result of the shift operation.

Examples –
1. This simple subtract instruction subtracts a value stored in register r2 from a value stored in register r1. The result is stored in register r0

PRE
r0 = 0x00000000     ;  As this register is a register to hold the output, that’s why it is empty before execution
           r1 = 0x000000002   ; register r1 holds the value ‘2’
           r2 = 0x000000001   ; r2 holds another value ‘1’
           SUB r0, r1, r2      ; r0=r1 – r2. Here the subtracted value (r0 – r1) is moved to r0 after performing operation.

POST
          r0 = 0x00000001  ; This is the output of above instruction moved to r0 register

2. This reverse subtract instruction (RSB) subtracts r1 from the constant value #0, writing the result to r0.
     Reverse subtraction is helpful for integer values so that the instruction can be subtracted with no complexity.

PRE
r0 = 0x00000000   ; Output register 
          r1= 0x00000077   ; value to be reverse subtracted
          RSB r0, r1, #0      ; Rd = 0x- – r1

POST 
r0 = -r1 = 0xffffff89   ; reversed output gets generated and stored in the register r0

Usage of barrel shifter with arithmetic instructions –
Barrel shifting is one of the powerful features of the ARM instruction set.
It pre processes one of the operand/ registers before performing operation on it.

Example –

PRE
r0 = 0x00000000   
           r1 = 0x00000005   
           ADD r0, r1, r1, LSL #1

POST
r0 = 0x0000000f   
           r1 = 0x00000005

2. Logical Instruction –
Logical instructions perform bitwise logical operations on the two source registers.
Syntax: <instruction>{<cond>} {S} Rd, Rn, N

AND	logical bitwise AND of two 32-bit values	Rd = Rn & N
ORR	logical bitwise OR of two 32-bit values	Rd = Rn | N
EOR	logical exclusive OR of two 32-bit values	Rd = Rn ^ N
BIC	logical bit clear (AND NOT)	Rd = Rn &~ N

Example –
1. This example shows a logical OR operation between registers r1 and r2, r0 holds the result.

PRE 
r0 = 0x00000000
          r1 = 0x02040608
          r2 = 0x10305070
          ORR r0, r1, r2

POST
r0 = 0x12345678

2. This example shows a more complicated logical instruction called BIC, which carries out a logical bit clear.

PRE
r1 = 0b1111
          r2 = 0b0101
          BIC r0, r1, r2

POST 
          r0 = 0b1010 

3. Multiply Instruction – 
The multiply instructions multiply the contents of a pair of registers depending upon the instruction, and accumulate the result along with another register . The long multiplies accumulate onto a pair of registers representing a 64-bit value. The final result is placed on a destination register or pair of registers.

Syntax –  MLA{<cond>}{S} Rd, Rm, Rs, Rn
              MUL{<cond>}{S} Rd, Rm, Rs

MLA	Multiply and accumulate	Rd = (Rm * Rs) + Rn
MUL	multiply	Rd = Rm * Rs

Syntax –  <instruction>{<cond>}{S} RdLo, RdHi, Rm, Rs

SMLAL	signed multiply accumulate long	[RdHi, RdLo] = [RdHi, RdLo] + (Rm * Rs)
SMULL	signed multiply long	[RdHi, RdLo] = Rm * Rs
UMLAL	unsigned multiply accumulate long	[RdHi, RdLo] = [RdHi, RdLo] + (Rm * Rs)
UMULL	unsigned multiply long	[RdHi, RdLo] = Rm * Rs

Processor implementation handles the number of cycles taken to execute a multiply instruction.

Example 1  –

• This example represents the simple multiply instruction that multiplies registers r1 and r2 together and places the result into a register r0.
• Register r1 is equal to the value 2, and r2 is equal to 2, then replaced to r0.

PRE 
          r0 = 0x00000000   ; register to hold the output
          r1 = 0x00000002   ; register holding the operand 1 value
          r2 = 0x00000002   ; register holding the operand 2 value
          MUL r0, r1, r2   ; r0 = r1 * r2

POST
          r0 = 0x00000004   ; output of the multiplication operation
          r1 = 0x00000002   
          r2 = 0x00000002   ;   operands

Example 2 –

PRE
          r0 = 0x00000000   
          r1 = 0x00000000
          r2 = 0xf0000002
          r3 = 0x00000002
          UMULL r0, r1, r2, r3   ; [r1, r0] = r2 * r3

POST 
          r0 = 0xe0000004  ; = RdLo
          r1 = 0x00000001  ; = RdHi

4. Comparison Instructions –
These instructions are used to compare or test a register with a 32-bit value. They update the cpsr flag bits according to the result, but do not affect other registers. After the bits have been set, the information can then be used to change program flow by using conditional execution.

Syntax – <instruction>{<cond>} Rn, N

CMN	compare negated	flags set as a result of Rn + N
CMP	compare	flags set as a result of Rn – N
TEQ	test for quality of two 32 – bit values	flags set as a result of Rn ^ N
TST	test bits of a 32-bit value	flags set as a result of Rn & N

N is the result of the shifter operation.

Example –

PRE 
          cpsr = nzcvqift_USER     
          r0 = 4   ; register to be compared
          r9 = 4  ; register to be compared
          CMP r0, r9

 POST 
cpsr = nzcvqift_USER   output generated after comparison

5. Move Instructions –
Move is the simplest ARM instruction. It copies N into a destination register Rd, where N is a register or immediate value. This instruction is useful for setting initial values and transferring data between registers.

Syntax – <instruction>{<cond>}{S} Rd, N

MOV	Move a 32-bit value into a register	Rd = N
MVN	move the NOT of the 32-bit value into a register	Rd = ~N

Example – 
PRE
          r5 = 5    ; register value 
          r7 = 8    ; register value
          MOV  r7, r5  ;let r7 = r5

POST 
          r5 = 5       ; data in the register after moving the r5 data into r7
          r7 = 5       ; output after  move operation

Barrel Shifter – 
It is a device to shift a word with variable amount. It is one of the logic devices used to pre-process one of the operand/register before operating on to the ALU operations. And it is one of the best features of ARM.

Barrel Shifter

Mnemonics	Description	Shift	Result	Shift amount
LSL	logical shift left	xLSLy	x<<y	#0-31 or Rs
LSR	logical shift right	xLSRy	(unsigned) x>>y	#1-32 or Rs
ASR	arithmetic right shift	xASRy	(signed) x>>y	#1-32 or Rs
ROR	rotate right	xRORy	((unsigned) x>>y) | (x<<(32 – y)	#1-31 or Rs
RRX	rotate right extended	xRRX	(c flag << 31) | ((unsigned) x>>1)	none

x represents the register being shifted and y represent the shift amount

N shift operations	Syntax
Immediate	#immediate
Register	Rm
Logical shift left by immediate	Rm, LSL #shift_imm
Logical shift left by register	Rm, LSL Rs
Logical shift right by immediate	Rm, LSR #shift_imm
Logical shift right by register	Rm, LSR  Rs
Arithmetic shift right by immediate	Rm, ASR #shift_imm
Arithmetic shift right by register	Rm, ASR Rs
Rotate right by immediate	Rm, ROR #shift_imm
Rotate right by register	Rm, ROR Rs
rotate right with extend	Rm, RRX

Example –

• This example of a  MOVS instruction shifts register r1 left by one bit.
• This multiplies register r1 by a value 2¹

PRE
          cpsr = nzcvqiFt_USER
          r0 = 0x00000000
          r1 = 0x80000004
MOVS   r0, r1, LSL #1

POST 
          cpsr = nzCvqiFt_USER
          r0 = 0x00000008
          r1 = 0x80000004

• The C flag is updated in the CPSR because the S suffix is present in the instruction mnemonics.