1. EEE305 Microcontrollers Week 12: A look at Assembler Ian McCrum

2. What I expect you to be able to do There are 35 simple instructions that a PIC can execute; you need not all of these, but you should be able to look up what they are. You should be able to work out what instruction causes a certain action to occur, e.g how to SET a bit, or how to MOVE a number from one place to another (actually most “MOVES” are actually “COPIES”) You should be able to convert a 14 bit number into a assembler instruction, and convert an assembler instrucition into a 14 bit code. You should be able to work out what binary pattern to put in a peripheral control register to configure it (e.g the timer or serial port)

3. The earliest computers were programmed by technicians writing binary codes just like this. As you can see, though, binary codes are very hard for human beings to read or write because they’re completely arbitrary; they look like gibberish. Another reason binary codes are hard to write is that many of them refer to locations in memory. For instance, a “go to” instruction will have to say what memory address to jump to. Programming would be much easier if you could label a location in the program and have the computer figure out its address. A PIC spends its time reading instructions from the program memory, one after another, and doing whatever these instructions say. Each instruction consists of 14 bits. If you could see the bits as binary ones and zeroes, a program might look like this: 11000000000000 00000001100110 11000000000001 00000010000110 10100000000100 Taken from http://www.ai.uga.edu/mc/microcontrollers/pic/picassem2004.pdf [accessed 1/5/13]

4. For both of these reasons, assembly language was invented over forty years ago. Or, to be more precise, many assembly languages have been invented, one for each type of CPU.What assembly languages have in common is that the instructions are abbreviated by readable codes (mnemonics) such as GOTO and locations can be represented by programmer-assigned labels. For example, in assembly language, the binary instructions just mentioned would be: movlw B’00000000’ tris PORTB movlw B’00000001’ movwf PORTB fin: goto fin In English: Put the bit pattern 00000000 into the W register and copy it to the tri-state control register for port B, thereby setting up port B for output; then put 00000001 into W and copy it to port B itself; and finally stop the program by going into an endless loop. This will light an LED wired up to RB0

5. ; assembly language program TURNON.ASM ; Turns on an LED connected to B0. ; Uses RC oscillator, about 100 kHz. ; CPU configuration processor 16f84 include <p16f84.inc> __config _RC_OSC & _WDT_OFF & _PWRTE_ON ; Program org 0 ; start at address 0 ; At startup, all ports are inputs. ; Set Port B to all outputs. movlw B’00000000’ ; w := binary 00000000 tris PORTB ; copy w to port B control reg ; Put a 1 in the lowest bit of port B. movlw B’00000001’ ; w := binary 00000001 movwfPORTB ; copy w to port B itself ; Stop by going into an endless loop fin: goto fin end ; program ends here

6. The architecture of the PIC

8. Program Memory Organization Data Memory Organization The data memory is partitioned into multiple banks which contain the General Purpose Registers and the Special Function Registers. Bits RP1 (STATUS<6>) and RP0 (STATUS<5>) are the bank select bits. Each bank extends up to 7Fh (128 bytes). The lower locations of each bank are reserved for the Special Function Registers. Above the Special Function Registers are General Purpose Registers, implemented as static RAM. All implemented banks contain Special Function Registers. Some frequently used Special Function Registers from one bank may be mirrored in another bank for code reduction and quicker access.

11. If the STATUS register is the destination for an instruction that affects the Z, DC or C bits, then the write to these three bits is disabled. These bits are set or cleared according to the device logic. Furthermore, the TO and PD bits are not writable, therefore, the result of an instruction with the STATUS register as destination may be different than intended Use only BCF, BSF, SWAPF and MOVWF instructions to alter the STATUS register.

12. INSTRUCTION SET SUMMARY Each PIC16F87X instruction is a 14-bit word, divided into an OPCODE which specifies the instruction type and one or more operands which further specify the operation of the instruction. The PIC16F87X instruction set summary in Table 13-2 lists byte-oriented, bit-oriented, and literal and control operations. Table 13-1 shows the opcode field descriptions. For byte-oriented instructions, 'f' represents a file register designator and 'd' represents a destination designator. The file register designator specifies which file register is to be used by the instruction. The destination designator specifies where the result of the operation is to be placed. If 'd' is zero, the result is placed in the W register. If 'd' is one, the result is placed in the file register specified in the instruction. For bit-oriented instructions, 'b' represents a bit field designator which selects the number of the bit affected by the operation, while 'f' represents the address of the file in which the bit is located. For literal and control operations, 'k' represents an eight or eleven bit constant or literal value.

14. All instructions are executed within one single instruction cycle, unless a conditional test is true or the program counter is changed as a result of an instruction. In this case, the execution takes two instruction cycles with the second cycle executed as a NOP. One instruction cycle consists of four oscillator periods. Thus, for an oscillator frequency of 4 MHz, the normal instruction execution time is 1 us. If a conditional test is true, or the program counter is changed as a result of an instruction, the instruction execution time is 2 us. There exists a couple of instructions called OPTION and TRIS. Microchip are removing these from their more modern chips, hence the warning in the datasheet; The key instructions are to move a number into a register and move the contents of a register to another register. The chip designer had to make compromises, so often you need to use several instructions to achieve what you want. The W register is an important register, it is usually involved in data transfers.

15. Examples from instruction set

16. Examples from instruction set And a BTFSC version is available as well, this is how “IF” statements are written in assembler

19. Example of “Hand” Assembly ; Given the program snippet below movlw B’00000001’ ; note that PORTB is at FSR address 0x06 (OHP 09) movwf PORTB Relevant parts of the list of instructions LITERAL AND CONTROL OPERATIONS BYTE-ORIENTED FILE REGISTER OPERATIONS Hence the first instruction becomes 11 0000 0000 0001 (assuming we use 00 for xx ) The second instruction becomes 00 0000 10000110 (since PORTB is 0x06 ) Check OHP 16 to glean understanding of what each instruction does.

20. As a Tut, try yourself to write down the assembler that does the following Q1. Move the contents of file register 0x08 into W Q2. Copy the contents of W into register 6 Q3. RESET bit 3 of PORTB Q4. SET bit 0 of PORTB Q5. In one instruction, RESET bits 4 and 5 of W Q6. In one instruction SET bits 0 and 1 of W Hints for Q5 and Q6, use the boolean AND or OR operations. Q7-Q12 convert each of the assembler instructions above to 14 bit binary machine code.