3. ALU and jumps

Rev 1.72Dr Pierre-André Mudry

pierre-andre.mudry@hevs.ch

Information systems - µC

Objectives

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ALU and jumps in code

Assembly programming techniques ALU flags and implementation Jumps and conditional execution

Disclaimer

The stack slides were adapted fromM. Bianchi's former course.

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ASSEMBLY PROGRAMMINGTECHNIQUES

How does it work?

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Instructions seen so far movwf f

• Move (W) to register f movf f, W

• Move register f to (W) movff fs, fd

• Move register (fs) to register (fd) movlw k

• Move literal value (an 8 bits number) to (W) goto k

• Load k (20 bits) into PC[20..1]. k usually corresponds to a label addlw k

• w (w) + k, where k is 8 bits addwf f,d

• d (w) + (f), where d can be either 0 or 1

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Brief overview http://www.mikroe.com/chapters/view/10/chapter-9-instruction-set/

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Destination bit d, 0 or 1 ?aka W or F

x equ 0x3

begin:movlw .5movff WREG, xmovlw .3addwf x, W

Skipping instructionsto implement an if

• Conditions are easy to implement in assembly skip next instruction if some condition is true

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Mnemonic EffectTSTFSZ f Test register (f), skip next instruction if content is zeroCPFSEQ f CPFSGT f CPFSLT f

Compare register (f) with WREG, skip if equal, > or <

DECFSZ f DCFSNZ fINCFSZ f INCFSNZ f

Increment/decrement (f), skip if == 0 (or skip if ≠ 0)

BTFSC f, x BTFSS f, x

Test bit X from register (f), skip if == 0 (or skip if ≠ 0)

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A simple loopbegin:

movlw .5

decrement:decf WREGtstfsz WREGgoto decrement

rest_of_code:…

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i equ 0x equ 1

movlw 5 ; The value to checkmovwf ibtfss i, 0 ; Check if even or oddgoto evengoto odd

even:movlw .10movwf xgoto loop

odd:movlw .20movwf xgoto loop

loop:goto loop

Adresses between 0 and 95 are located in data memory

2. THE ALUDive deep inside

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Organisation

• What are all those things?

• Registers withspecialfunctions…

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Source: Microchip PIC18F6680 datasheet

ALU in detail

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Exercise 3.0

• What is the content of the first byte of memory (which contains the value 10 at the beginning) and of Wafter the following code:

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movlw 3addwf 0x00, 1

ALU presented is not 100% complete

• More stuff is required for multiplication Why ?

• Comparisons• …

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Word of the wise : "Even the simplest thing can be complexto implement."

The STATUS register

• Our first SFR in detail!• Modified by the ALU as the result of

an instruction. Don't write there!• Not every bit modified by every

operation

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STATUS bits description

• N

• OV

• Z

• DC

• C

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STATUS modificationclrf WREGmovlw 0x1addlw 1movlw 0x80addlw 0movlw 0x7faddlw 1movlw 0xffaddlw 1

The STATUS register (2)

• Serves for implementing if in higher language, conditions values comparisons out of range computations

• Coding techniques will be discussed in detail later!

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Operations NOT affecting status

• Some move operations movlw, movwf, movff

• Branch operations• Increment/decrement with skip incfsz, incfsnz

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Exercise 3.1

• Consider that WREG = 1, address a = 255 All status flags are equal to 0.

• What is the status of C, Z, OV, N afterthe following (independent) operations?1. clrf WREG2. movlw .53. subwf a4. addwf a

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Exercise 3.2

• With the instructions seen so far, how can youcheck that memory locations 0x10 and 0x21have the same content? Explain with code

• toto is a byte declared in memory. What's the purpose of this operation:

movf toto, 1

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Remark: values at power-up / reset

• The content of the data memory (GPR) at power-up/reset is UNKNOWN.

• The content of the SFRs (specialregisters, such as TRISA, STATUS, PORTA, …) is … complicated One must look at the datasheet

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Serie 1

JUMPS AND FUNCTION CALLSChanging the instruction flow

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The program counter register (PC)

• Contains the address of nextinstruction that will be executed

• Serves for loops / function calls• Width of the PC: • Reason:

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Program adresses

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PC characteristics

• Several ways to update it 1. Automatic update (every cycle, +2) 2. Load a value into it

• Problem : 21 bits is much more than 8 bit (datapath) !

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Source: Microchip

PIC18F6680 datasheet

Accessing PC contents

• PC is composed of PCL: the lower byte, writable. Last bit

set to 0 in hardware PCH: higher byte, accessible trough the

PCLATH register PCU: the upper byte, accessible

throught the PCLATU

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Relative and absolute jumps

• Absolute jumps (absolute value loaded into PC) PC absolute value

• Relative jumps (relative to currentPC address), PC PC + offset

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Situation 1- GOTO instructionJumping anywhere

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var1 equ 0x10var2 equ 0x21

movlw 0x1movwf var1

loop:incf var1, fgoto loop

0000 0E01 MOVLW 0x1 movlw 0x10002 6E10 MOVWF 0x10, ACCESS movwf var10004 2A10 INCF 0x10, F, ACCESS incf var1, f0006 EF02 GOTO 0x4 goto loop0008 F000 NOP

Source: Microchip PIC18F6680 datasheet

Situation 2 - Relative branchJumping near the current location

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var1 equ 0x10var2 equ 0x21

movlw 0x1movwf var1

loop:incf var1, fbra loop

0000 0E01 MOVLW 0x1 movlw 0x10002 6E10 MOVWF 0x10, ACCESS movwf var10004 2A10 INCF 0x10, F, ACCESS incf var1, f0006 D7FE BRA 0x4 bra loop

• Addresses computedby the assembler

• Can be negativeaddresses (literal issigned, 11 bits)

• Various forms: branch if (zero, negative, …)

Function callsAssembler perspective

• Until now, we did not have function calls.• A function (without parameter at the

moment) should

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Internal stack

• The PIC18F has a stack• Is a special data structure to store

data.• Operations: PUSH, put something on top POP, remove something from top

• LIFOP.-A. Mudry ALU and jumps 35

The stack

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Function calls (2)Assembler perspective

• Special instruction for this : call Stores the return PC address

onto the stack Jumps to an address with

goto Stores WREG, STATUS, BSR

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Source: Microchip PIC18F6680 datasheet

Function calls (3)Assembler perspective

• When function is over return Restores the PC Restores WREG, STATUS, BSR (is FAST mode is used)

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Source: Microchip PIC18F6680 datasheet

Fast modeFor calling sub-routines

• Allows to store a copy of WREG, STATUS and BSR register.

• Limited Only one level Can't be used in nested calls Does not work with interrupts

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Function calls (4)

• Return values? Could use WREG, but limited

• Arguments? Could use WREG, but limited

• Recursion Very limited (max 31 levels deep)We will let the compiler do this later

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What we’ve seen in this lecture

• How the ALU works• How are jumps and calls implemented• YOUR JOB FOR NEXT LESSON: Read in PIC18 datasheet, available on

http://sin.begincoding.net/documentation/• Page 78 (reset state of registers)• Pages 93-97 (instructions encoding)

What's next?Instruction pipeline, IOs

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