General issues. Format MACROS. Example 1: Extract MS nibble of register A as Lsnibble of Reg. B 7/11/2014

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1 General issues MACROS Set of instructions grouped under one user-defined mnemonic Macro is expanded by assembler (does not save memory) Accept parameters Labels must be declared local A macro must be defined before it can be used Format MacronameLabel MACRO [arguments] LOCAL: labelnames (if necessary) instructions go here ENDM Example 1: Extract MS nibble of register A as Lsnibble of Reg. B ExtractNibble MACRO A, B, C ; uses C as counter Local msbittransfer clr B ; clean receptor mov #4, C ; initialize counter msbtransfer: rla A ; msb of A to carry rlc B ; introduce as B s lsb dec C ; go for next bit jnz msbtransfer ; until B(12) ENDM Comment: after the macro, A has been shifted left four bits. 1

2 Example 2. Multiply by 10 Notice that 10A = 8A + 2A Mult10 MACRO A ; 0< A <6553 or < A < 3276 add A,A ; 2A push A ; 2A is stored add A,A ; 4A add A,A ; 8A ; 10A, pop emulated ENDM ** Question: Why the bounds? What do you need to relax these bounds? Example 3: Using the defined macros Decimal to binary conversion Four digit Decimal to binary (hex) conversion A3A2A1A0 N = 10 3 A A2 +10 A1 + A0 Decimal number coded as BCD in R5 Conversion obtained in R6. Use R15 for global counter Use R7 for intermediate extractions. Based on synthetic division (Hoeners multiplication) P = A3, for j=2 to 0 do { P 10P + A(j) } N = P0 Example 3: Code mov # 0x, R5 ; is the ; BCD code of number StartConv: clr R6 ; ExtractNibble R5, R6, R8 ; N=R6=A3 mov #3,R15 SynTDiv Mult10 R6 ; N=10N ExtractNibble R5,R7, R8 ; Extract Aj add R7,R6 ; N = 10N + Aj dec R15 ; If not extracted jnz SyntDiv ; and Add A0, repeat EndConv ; conversion in R6 SUBROUTINES 2

3 Definitions: Subroutines (functions) are pieces of code separated from main code. They are invoked with instruction call from another place main code or other subroutine-, Finish with instruction ret. (Return from subroutine) The call instruction proceeds in two steps: Pushes PC to save address of instruction following the call Loads PC with entry instruction of subroutine Instruction ret pops the PC Hints for subroutines As short as possible For reusable subroutine codes, save all registers in code not used as passing parameters resources Think of them as local registers (local variables) Save pushing, and retrieve popping in reverse order. Example ThisSub: push R8 push R pop R9 pop R8 ret Hints for subroutines (2) Only one task Avoid calling another subroutine from within except if absolutely necessary Test subroutine independently Write a program whose only purpose is to call the subroutine Add subroutines one at a time to test compatibility Passing parameters Parameters are data provided to or returned from subroutines There are several methods. Most common: By register By Memory By Stack Combination Not all subroutines need to pass parameters 3

4 Example 4: Register passing ; This sub is used to multiply a word N by 10. Since 10 = 5x2, the number is ; multiplied by 5 and then by 2. ; The initial word should be less than 6500 (1964h) so that result is a word size number. ; Data is passed to subroutine in R6, and result is returned in R6. ; R15 is used as local variable Example 4: cont Using the subroutine (In main code or calling subroutine): SMALL_MUL_TEN push R15 ; save R15 for general use, local variable mov R6, R15 ; R15=N rla R6 ; multiply by 2 == >R6= 2N rla R6 ; R6 = 4N add R15, R6 ; R6= 5N rla R6 ; R6 = 10 N pop R15 ; Restore R15 ret mov #N,R6 ; prepare data ThisCall call #SMALL_MUL_TEN ; To get R6= 10N AfterCall ;product in R6 for further use Example 4b: Passing through memory ; This sub is used to multiply a word by 10. Since 10 = 5x2, the number is ; multiplied by 5 and then by 2. ; The initial word should be less than 6500 (1964h) so that result is a word size number. ; Data is stored in memory, at location DATAforM, and result is returned to memory ; location Product10M. ; R15 is used as local variable SMALL_MUL_TEN push R15 ; save R15 for general use, local variable mov &DataforM, &Product10M ; [Product10M] = N mov &DataforM, R15; R15= N rla & Product10M ; [Product10M] = 2N rla & Product10M ; [Product10M] = 4N add R15, & Product10M ; [Product10M] = 4N rla & Product10M ; [Product10M] = 10N pop R15 ; Restore R15 ret In data memory: DataforM DS16, 1 Product10M DS16,1 MyWords DS16, 15 Example 4b: cont Using the subroutine (In main code or calling subroutine): mov #N,&DataforM ; prepare data ThisCall: call #SMALL_MUL_TEN ; AfterCall: ;product at &Product10M 4

5 Example 4c: Passing through stack ; This sub is used to multiply a word by 10. Since 10 = 5x2, the number is ; etc. ; Data has been pushed before calling subroutine. Result is returned to same ; address (if desired through a register; See next slide) ; R15 is used as local variable SMALL_MUL_TEN push R15 ; save R15 for general use, local variable mov 2(SP), R15; R15= N stored at [SP+2] rla 2(SP) ; [SP+2]= 2N rla 2(SP) ; [SP+2] = 4N add R15, 2(SP) ; [SP+2] = 5N rla 2(SP) ; [SP+2] = 10N pop R15 ; Restore R15 ret Example 4b: cont Using the subroutine (In main code or calling subroutine): push #N, ; prepare data ThisCall: call #SMALL_MUL_TEN ; AfterCall: ;product available with pop *********************************** Important note: If you return product in a Register, then the instruction after call must be incd SP to clean stack and avoid depletion. NOTES ON USING THE ASSEMBLER General Remarks Try to follow a similar pattern in your programs (this will help in planning) Document your program. Proper name to constants and variables can help in commenting Use labels at strategic instructions to help reading. 5

6 An absolute code example A relocatable code example Too many directives Introduction to some IAR directives and operators 6

7 continuation And more More. Absolute and Relocatable sections PLC: Program Location Counter Used by linker to write in memory Controlled by directives ORG <Memory Address > ALIGN <number> Symbol $ An absolute section works with ORG An absolute code works with ORG in all its segments 7

8 Relocatable Segments Uses one location counter for each segment, Section Location Counters. All initialized at 0. Value is an offset value ORG $+value clears value bytes from current location Segment is placed in memory by LINKER RSEG Begins a relocatable segment When using relocatable stack segment Declare Initialize RSEG type Segment Directives DATA for a data segment (in RAM) CODE for code segment (in ROM) CSTACK for THE stack at the top of RAM <Name of Segment> STACK for stack-like segments SFE(XX) and SFB(XX) stand for the last and first addresses of segment XX Memory Allocation Initialized data: data stored in memory with a specific given value at the time of compilation Uninitialized data: Memory space reserved without particular values 8

9 Data allocation directives Meaning for initialized cases The format for uninitialized data is: [LABE] <Directive> N where N is the number of spaces being separated Important NOTE:.double is not working for MSP430 compilation, but works for other embedded systems. 9

General issues MACROS

General issues MACROS MACROS General issues Set of instructions grouped under one user-defined mnemonic Macro is expanded by assembler (does not save memory) Accept parameters Labels must be declared local A macro must be defined