1 / 12

ELEC 2220 Computer Systems

ELEC 2220 Computer Systems. Chapter 4. Modular Programming Soo-Young Lee Department of Electrical and Computer Engineering Auburn University. 4. Modular Programming: Chapter Objectives. Subroutine Subroutine-related instructions Parameter Passing. 4-1. Subroutines.

azra
Download Presentation

ELEC 2220 Computer Systems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ELEC 2220 Computer Systems Chapter 4. Modular Programming Soo-Young Lee Department of Electrical and Computer Engineering Auburn University ELEC2220 Auburn University

  2. 4. Modular Programming: Chapter Objectives • Subroutine • Subroutine-related instructions • Parameter Passing ELEC2220 Auburn University

  3. 4-1. Subroutines A large task may be partitioned into blocks, and some of the blocks can be implemented as subroutines (modules), especially those executed repeatedly. • Example • Finding the sum of all elements in array[N] • Issues: task partitioning, parameter passing Main • Why modular programming? • easily understand • easy debugging • localized changes • share modules EA of array, N Sum Sum up elements in array[N] Subroutine ELEC2220 Auburn University

  4. 4-1. Subroutines • Task Partitioning • Define the computation to be carried out by each subroutine. • Parameter Passing • Define the input parameters (constants, variables, etc.) to be passed to the subroutine. • Define the results from the subroutine. • How are they passed between the calling procedure and the subroutine? ELEC2220 Auburn University

  5. 4-1. Subroutines • Example N equ 10 org $800 array ds.b 10 total ds.b 1 org $8000 … ldx #array ldab #N jsr sum staa total …. rts • sum • ldaa #0 • next: • adda 1,x+ • dbne b,next • rts ELEC2220 Auburn University

  6. 4-2. Subroutine Instructions • BSR (branch to subroutine) • Save the return address (address of the instruction immediately following the BSR) and initiate execution of the subroutine. • Saving return address • SP  SP – 2 • M[SP]:M[SP+1]  return address • Initiate execution of the subroutine • PC  PC + distance ; relative addressing • JSR (jump to subroutine) • The same as BSR except for the addressing mode • Initiate execution of the subroutine • PC  address of the subroutine : all other addressing modes ELEC2220 Auburn University

  7. 4-2. Subroutine Instructions • RTS (return from subroutine) • Retrieve the return address from the stack • PC  M[SP]:M[SP+1] • SP  SP + 2 • Never jump to or out of a subroutine using branch instructions ! • Don’t forget to initialize SP ELEC2220 Auburn University

  8. 4-2. Subroutine Instructions • Subroutine Sequence … Jsr subr next instr ; addr_of_next …. subr first instr ; addr_of_first …. rts …. 1. Save PC Addr_of_next Addr_of_next SP SP Stack Stack 3. Return PC  addr_of_next 2. Initiate execution of subr PC  addr_of_first ELEC2220 Auburn University

  9. 4-3. Parameter Passing 4-3-1. Registers • Data to be processed are passed in registers to the subroutine. • Results are returned in registers to the calling procedure. • The number of parameters is limited. 4-3-2. Memory • The global shared memory variables are used for parameter passing. • Subroutine: variable-name specific 4-3-3. Stack • Parameters are passed to and from the subroutine via the stack. • Data are pushed onto the stack, which are popped by the subroutine. • Results are pushed by the subroutine, which are popped by the calling procedure. • Careful with the return address on the stack ELEC2220 Auburn University

  10. 4-3. Parameter Passing 4-3-4. Address Passing • Addresses of parameters, instead of their values, are passed. • Subroutine: variable-name independent • Particularly useful when the size of data to be passed is large. • Example: finding the maximum in an array Find_max( aptr, N) Int *aptr, N; { Int temp_max, i; temp_max = *aptr; for(i=1; i<N; i++) if( *(aptr+i) > temp_max ) temp_max = *(aptr+i); return(temp_max); } int array[100], max; N=100; max = find_max(array, N); …. ELEC2220 Auburn University

  11. 4-5. Macro • A subroutine may be too costly for a small task due to the call/return overhead. • A small task (code) can be defined as a macro which may be considered as a user-defined instruction. • An assembler copies the code wherever it encounters the macro. • The program flow doesn’t change. • The actual program length after assembled is longer than what it appears to be. • Example (CodeWarrior) sum_abs MACRO ldaa \1 adda \2 bge done\@ nega done\@ ENDM movb #$90,var1 Movb #$30,var2 sum_abs var1,var2 ELEC2220 Auburn University

  12. 4-6. Chapter Summary • Modular programming has several advantages. • Task partitioning • BSR, JSR • Save the return address and load PC with the address of the subroutine • RTS • Parameter Passing • Registers • Memory • Stack • Address passing • MACRO ELEC2220 Auburn University

More Related