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Chapter 7 Linear Assembly

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  1. Chapter 7 Linear Assembly

  2. Learning Objectives • Comparison of programming techniques. • How to write Linear Assembly. • Interfacing Linear Assembly with C. • Assembly optimiser tool.

  3. Introduction • With the assembly optimiser, optimisation for loops can be made very simple. • Linear assembly takes care of the pipeline structure and generates highly parallel assembly code automatically. • The performance of the assembly optimiser can easily reach the performance of hand written assembly code.

  4. Comparison of Programming Techniques C C ++ Optimising Compiler 80 - 100% Low LinearASM Assembly Optimiser 95 - 100% Med ASM Hand Optimised 100% High Source Efficiency* Effort * Typical efficiency vs. hand optimized assembly.

  5. Writing in Linear Assembly ZERO sum loop LDH *p_to_a, a LDH *p_to_b, b MPY a, b, prod ADD sum, prod, sum SUB B0, 1, B0 B loop • Linear assembly is similar to hand assembly, except: • Does not require NOPs to fill empty delay slots. • The functions units do not need to be specified. • Grouping of instructions in parallel is performed automatically. • Accepts symbolic variable names.

  6. How to Write Code in Linear Assembly .cproc defines the beginning of the code _sa_Function .cproc ZERO sum loop LDH *pm++, m LDH *pn++, n MPY m, n, prod ADD sum, prod, sum [count] SUB count, 1, count B loop .return sum .endproc .return specifies the return value .endproc defines the end of the linear assembly code • File extension: • Use the “.sa” extension to specify the file is written in linear assembly. • How to write code: NO NOPs required NO parallel instructions required NO functional units specified NO registers required

  7. Passing and Returning Arguments int y = dotp (short* a, short* x, int count) _dotp .cproc pm, pn, count ... .return y .endproc • “pm” and “pn” are two pointers declared in the C code that calls the linear assembly function. • The following function prototype in C calls the linear assembly function: • The linear assembly function receives the arguments using .cproc:

  8. Declaring the Symbolic Variables .reg pm, pn, m, n, prod, sum • All the symbolic registers except those used as arguments are declared as follows: • The assembly optimiser will attempt to assign all these values to registers.

  9. Complete Linear Assembly Code _dotp .cproc pm, pn, count .reg m, n, prod, sum ZERO sum loop LDH *pm++, m LDH *pn++, n MPY m, n, prod ADD sum, prod, sum [count] SUB count, 1, count B loop .return sum .endproc • Note: Linear assembly performs automatic return to the calling function.

  10. Function calls in Linear Assembly Function1.sa _function1 .cproc a, b .reg y, float1 MPY a,b,y .call float1 = _fix_to_float(y) .return .endproc Fix_to_float.sa _fix_to_float .cproc fix .reg float1 INTSP fix, float1 .return float1 .endproc • In linear assembly you can call other functions written in C, linear assembly or assembly. • To do this the .call directive is used: • Note: Branch out of a linear assembly routine is not allowed.

  11. Invoking the Assembly Optimiser • The development tools recognise the linear assembler code by the file extension “.sa”. • The assembly optimiser uses the same options as the optimising C compiler. • Note: In CCS you can change the options of each file individually by right clicking on the file in the project view and selecting “File Specific Options…”.

  12. Linear Assembly Examples • The following chapters have code written in linear assembly: • \Code\Chapter 15 - Infinite Impulse Response Filters • \Code\Chapter 17 - Goertzel Algorithm • For more details on Interfacing C and Assembly see Chapter 11.

  13. Chapter 7 Linear Assembly - End -

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