L04 - AVR CPU Registers

Microcontrollers and AVR Specific Information

CPU Registers

1: General Purpose Registers
2: Status Register (SREG) – contains info about result of last executed instruction
3: Program Counter (PC) – Address of next instruction
4: Instruction Register (IR) – Holds fetched instruction
5: Stack Pointer (SP) – Points to top of stack

Stack

• The Stack is mainly used for storing temporary data for

  • local variable
  • return addresses after interrupts and subroutine calls.

• Implemented as growing from higher to lower memory locations.

  • Initial Stack Pointer is set equal to the last address of the internal SRAM

• PUSH command will decrease the SP.

• POP command will increase the SP.

Note Table 6-1. from data sheet has all assembly Stack Pointer instructions.

Stack Pointer - SP

• Points to top of the stack
  • Implemented as two 8-bit registers
• What does address in SP tell us?

SP implementation in AVR

Status Register SREG

Status Register (SREG):

• contains information about the result of the most recently executed instruction
  • can be used to altering program flow via conditional operations
• save/restore must be handled by software for interrupt calls (not automatic in hardware)

1. Bit7:I: Global Interrupt Enable. Setting this bit enables all the interrupts. Resetting this disables all interrupts.
2. Bit6:T: Bit Copy Storage. Used with BLD (bit load) and BST (bit store) ISTR for loading and storing bits from one register to another.
3. Bit5:H. Half Carry Flag. Indicates half carry in some arithmetic ISTRs.
4. Bit4:S. Sign Flag. This bit is the exclusive OR between the negative flag N and the Overflow flag V.
5. Bit3:V. Two's Complement Overflow Flag.
6. Bit2:N. Negative Flag.
7. Bit1 :Z. Zero Flag. Indicates a zero result after an arithmetic or logical operation.
8. Bit0:C. Carry Flag. Indicates a carry in arithmetic or logical operation.

Register File

• Most instructions have direct access to all registers
  • e.g. LD (load indirect from data space), MUL (multiply)
• Some only operate on R16-R31
  • LDI (load immediate), SBR/CBR (set/clear bits in register), ANDI, CPI, SUBI, SUBIC,MULS
• A few operate only on R16-R23
  • Special multiply operations MULSU, FMUL, FMULS, FMULSU
• Double word operations operate on register pairs (R24-31)
  • R25:R24,R27:R26,R29:R28,R31:R30
  • like ADIW, SBIW (Add/Subtract Immediate from Word)
• Most single-byte register or register+immediate operations are single cycle instructions
  • MUL is one exception.

Special Purpose Registers – X, Y, Z

• R26-R31 have special purpose
  • In addition to allowing general purpose usage
• 16-bit address pointers for addressing data space
• Used in functions for different addressing modes
  • Fixed displacement
  • Automatic increment/decrement