Instructions

Arithmetic

• ADD a, b, c: a = b + c
• SUB a, b, c: a = b - c
• Immediate Operands
  • For adding constants (notated as #NUM) to a register
  • Allows you to store a small constant inside the instruction to prevent a load operation
  • Add an I after the instruction (don’t need this for ARMv8)
  • Ex: ADDI X20, X22, #4: X20 = X22 + 4
• Treats everything as signed

Logical

• AND a, b, c: a = b & c
• ORR a, b, c: a = b | c
• EOR
  • Can be used as NOT when one input is entirely 1s
• LSL a, b, c: a = b << c
  • Multiply by $2^c$
• LSR a, b, c: a = b >> r
  • Divide by $2^c$
• Immediate operands can be used here in the same way
• Treats everything as unsigned

Transfer

• MOV XA, XB: Assigns XB to XA
• MOV XA, #B: Assigns B to XA

Load/Store

• LD_R__ [XA, #0], b: Load from Memory[b] to register a
• ST_R_ a, [XB, #0]: Store from register a to Memory[b]
• First Blank
  • Nothing: offset given as multiple of 4
  • U: Unscaled offset (offset is not given as multiple of 4)
  • X: Atomic
• Second Blank (For Load)
  • Nothing: Zero-extend loading style
  • S: Sign-extend loading style (negative numbers are padded with 1)
• Third Blank
  • Nothing: Doubleword (8 bytes)
  • H: Halfword (4 bytes)
  • B: Byte

Conditional Branch

• Explicit: Checks if register is 0
  • CBZ r, L1: if r == 0, go to label L1
  • CBNZ r, L1: if r != 0, go to label L1
• Implicit: Checks flags set
  • Written B.cond L1: if cond flag set, go to label L1

Comparison Flags

• Flags that can be set by
  • Using the dedicated CMP a, b instruction
  • Adding the S suffix to an arithmetic operation

Flags

Property	Flag
Negative	N
Zero	Z
Carry	C
Overflow	V

Comparisons (uses boolean operation on the flags):

Property	Signed	Unsigned
Equal	EQ	EQ
Not Equal	NE	NE
Less than	LT	LO
Less than or equal	LE	LS
Greater than	GT	HI
Greater than or equal	GE	HS

Trick: $0\le x<y$ can be done using a single unsigned comparison because a signed negative number would appear as a massive unsigned positive number

Unconditional Branch

• B L: Jump to label
• BR X30: Go to address based on value of X30
  • BR = branch to register
• BR LR: Return to caller
• BL L1: Branch and link to L1
  • Used for procedure call
  • Before jumping, saves the address of the next instruction into X30 (special register)
    • Node: every instruction if 4 bytes
  • Also saves the current context of X30 to the stack so it can be restored after that procedure returns
• Procedure Call Steps
  1. Place parameters in registers X0 to X7 (special case if more than 8 parameters)
  2. Transfer control to procedure
  3. Acquire storage for procedure
  4. Preform procedure’s operations
  5. Place result in register for caller (X0)
  6. Return to place of call (address in X30)
• Leaf procedure: procedure that does not call any other procedures
• When a procedure calls another procedure, saved registers are pushed to the stack and then restored after the procedure returns
  • Saved registers 19-25
  • Stack pointer X28
  • Frame pointer X29
  • Link (return address) X30