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Flexible Op2

• MOV Rd, op2 ; op2 in two operand MOV instruction
• ADD Rd, Rs, op2 ; op2 in 3 operand instruction
• CMP Rs, op2 ; op2 in 2 operand copare instruction

Data processing instruction op2 has a number of formats.

Op2 format	Examples	Number used in DP instruction
Register	MOV R0, R1	The value of a register
Literal	ADD R0, R1, #22	An immediate numeric literal
Shift	MVNS R0, R1, LSL #3	The value of a register shifted by a constant
Register-valued Shift	MOV R0, R1, ASR R2	The value of a register shifted by the number in another register
RRX	MOV R0, R1, RRX	RRX does not use a literal and always makes a value rotated right by 1

Notes

• MOV instructions with shift/rotate can use the shift as a short-form alias:
  • MOV R0, R1, ASR #4 -> ASR R0, R1, #4
  • MOV R3, R3, RRX -> RRX R3,R3
• Register-valued shifts Rx, LSL Ry. The LSL code can be any of LSL, LSR, ASR, ROR. In all cases Ry(4:0) determine the shift/rotate number of bits.
• Rm, RRX. is a 33 bit rotate right in which Rm(0) -> C -> Rm(31), and all other bits of Rm shift right by 1.
• Note that for all shifted op2 MOV R0, R1, LSL #3 the shifted register e.g. R1 does not change. Its value, shifted, is used as the op2 in the instruction operation. This value is not written back into R1.
• Valid shift/rotate numbers are in range #1 - #31.

Numeric Literals

• Allowed immediate literal values:
  • Any positive value <= 255. e.g. #22
  • For most instructions any negative value >= -255. E.g. #-22.
  • Literals may be written in binary #0b1011 or hexadecimal #0x6a.
  • Combinations of constant symbols and operators: #DATA1+16. #17-0x40

Carry written by flexible op2

When carry is not written by arithmetic, and S suffix is present, C (but not V) may be written by op2 in the follwing cases:

• For MOVS,MVNS,ADDS,ORRS,EORS,BICS, with shifts or rotates (including RRX) the bit last shifted or rotated out sets C. For ADDS etc the arithmetic Carry out is defined to set C and takes priority over this.
• A limited number of literal values larger than 8 bits are formed by rotating an 8 bit number N: 0 < N < 256 by 2r bits where 1 <= r <= 15. In this case for MOVS,MVNS,ADDS,ORRS,EORS,BICS instructions C is set by the last bit rotated out from the literal rotation. Note in particular that therefore all numbers with single bits set to 1 are valid literals.