#16: AVR Instruction Encoding

Reading

“AVR Instruction Set,” Section 6.4 “General Purpose Register File,” and Section 7.3 “SRAM Data Memory” in document doc0856 “The Program and Data Addressing Modes.”

Instruction Set Mapping

The Instruction Set of our AVR processor can be functionally divided (or classified) into: Data Transfer Instructions, Arithmetic and Logic Instructions, Bit and Bit-Test Instructions, Control Transfer (Branch) Instructions, and MCU Control Instructions.

While this functional division helps you quickly find the instruction you need when you are writing a program; it does not reflect how the designers of the AVR processor mapped an assembly instruction into a 16-bit machine instruction. For this task a better way to look at the instructions is from the perspective of their addressing mode. We will divide AVR instructions into the following addressing mode types.

Data Addressing Modes

• Direct Register Addressing, Single Register
• Direct Register Addressing, Two 32 General Purpose Registers Rd and Rr
• Direct Register Addressing, Two 16 and 8 General Purpose Registers Rd and Rr
• Direct I/O Addressing (including SREG)
• Direct I/O Addressing, First 32 I/O Registers
• Direct SRAM Data Addressing
• Immediate 8-bit Constant
• Immediate 6-bit and 4-bit Constant
• Indirect SRAM Data Addressing with Pre-decrement and Post-increment
• Indirect Program Memory Addressing (Atmel Program Memory Constant Addressing)

Control Transfer

• Direct
• Relative, Unconditional
• Relative, Conditional
• Indirect

MCU Control Instructions

ATmega328P Operand Locations

When selecting an addressing mode you should ask yourself where the operand is (data) located within the AVR processor.

DATA ADDRESSING MODES

DIRECT REGISTER ADDRESSING, SINGLE REGISTER

DIRECT REGISTER ADDRESSING, TWO OF 32 8-BIT GENERAL PURPOSE REGISTERS RD AND RR

Multiply

DIRECT I/O ADDRESSING (INCLUDING SREG)

DIRECT SRAM DATA ADDRESSING

IMMEDIATE

INDIRECT SRAM DATA WITH DISPLACEMENT

INDIRECT SRAM DATA ADDRESSING WITH PRE-DECREMENT AND POST-INCREMENT

INDIRECT PROGRAM MEMORY ADDRESSING (ATMEL PROGRAM MEMORY CONSTANT ADDRESSING)

CONTROL TRANSFER

DIRECT

All control transfer addressing modes modify the program counter.

INDIRECT

RELATIVE

MCU CONTROL INSTRUCTIONS

PROGRAM DECODING – WHO AM I?

Addr  Machine Instruction

Who_Am_I #1:
0204 9a5d      ____ ____, ____ // I/O direct
0205 985d      ____ ____, ____ // I/O direct
0206 9508      ____

Solution

Who_Am_I #2:
01f8 934f      ____ ____ // Indirect SRAM Data Addressing
01f9 b74f      ____ ____, ____ // I/O Direct
01fa 930f      ____ ____ // Indirect SRAM Data Addressing
01fb 9180 0103 ____ ____, ____ // Direct SRAM Data Addressing
01fd 9100 0102 ____ ____, ____ // Direct SRAM Data Addressing
01ff 2380      ____ ____, ____ // Direct Register Addressing,
0200 910f      ____ ____ // Indirect SRAM Data Addressing
0201 bf4f      ____ ____, ____ // I/O Direct
0202 914f      ____ ____ // Indirect SRAM Data Addressing
0203 9508      ____

Solution

PROGRAM ENCODING – DISPLAY

display:

:
_________      lds work0, imageR
_________      lds spi7SEG, imageD
_________      or spi7SEG, work0
_________      call spiTx
:
_________      ret

Solution

PROGRAM ENCODING – TURN LEFT

; ————————–
; ——- Turn Left ——–
turnLeft:

_________      push reg_F
_________      in reg_F,SREG
:
_________      lds work0, dir // x = work0 bit 1, y = work0 bit 0
_________      bst work0,0    // store y into T
_________      bld work1,1    // load dir.1 from T (dir.1 = y)
_________      com work0      // store /x into T
_________      bst work0,1
_________      bld work1,0    // load dir.0 from T (dir.0 = /x)
_________      sts dir, work1
:
_________      out SREG, reg_F
_________      pop reg_F
_________      ret

Solution

PROGRAM ENCODING – IN FOREST AND SPITXWAIT

inForest:
Address Machine Instruction
0131   _____   ldi ZL,low(table<<1) // load address of look-up
:
02e8   _____   lds work0, row // SRAM row address = 0101
02e9
02ea   _____   cpi work0, 0xFF
02eb   _____   breq yes
02ec   _____   clr cppReg // Compare to eor cppReg, cppReg
02ed   _____   rjmp endForest
yes:
02ee   _____   ser cppReg // compare to ldi cppReg, 0xFF
endForest:
:
02f3   _____   ret

Solution

spiTxWait:
0112   _____   in work0,SPSR
0113   _____   bst work0,SPIF
0114   _____   brtc spiTxWait
0115   _____   ret

Solution

PROGRAM ENCODING – BCD TO 7-SEGMENT DISPLAY

• Program Memory Indirect is great for implementing look-up tables located in Flash program memory – including decoders (gray code → binary, hex → seven segment, …)
• In this example I build a 7-segment decoder in software.

BCD_to_7SEG:
Address Machine Instruction
0131   _____   ldi ZL,low(table<<1) // load address of look-up
0132   _____   ldi ZH,high(table<<1)
0133   _____   clr r1
0134   _____   add ZL, r16
0135   _____   adc ZH, r1
0136   _____   lpm spi7SEG, Z
0137   _____   ret
0138   _____   table: DB 0b01111110, 0b0110000, 0b1101101 …

Solution

PROGRAM DECODING – SRAM INDIRECT

• Write and encode a program to set to ASCII Space Character (0x20), all the bytes in a 64-byte Buffer.

Appendix