CM0506 Exploring the LPC4088 Instruction Set: 4b

Transcription

1 CM0506 Exploring the LPC4088 Instruction Set: 4b Michael Brockway February 13, Introduction This lab activity continues exploring subroutine calling, this time looking at C or C++ code calling subroutines written in assembly language. Important You are expected to write the work you do in this lab up throughly as a short 2-3 pages) technical report. This piece of work will evenually become part of the second assessment for the module. It is important that You do the bulk of the work outline in this lab sheet during the relevant lab session; taking notes as you go, so that you can write it up as soon as possible afterwoods. You should not assume you will be able to do this work at the last minute. Out-of-hours access to the lab may be restricted; you need to make good use of the lab sooner rather than later. 1

2 2 Your tasks You are asked to write up your results to the following tasks in the suitably well organised and structured report of two-four pages. This will eventually become part of this semester s assessment for the module. 2.1 Calling the factorial function from C This exercise uses a Microvision project you will find in Lab4Srcs.zip which you downloaded and unzipped for that last lab activity. Find CAsmEx.zip and unzip it. You will find a MicroVision project containing two source files, both actually part of the project: main.c and factorial.s. The latter is slightly different from the factorial.s of the last lab as it is not a complete program, just the fact function. main.c 1: int fact(int x); 2: int x, y; 3: 4: int main() { 5: x = 4; 6: y = fact(x); 7: return 0; 8: } factorial.s 1: PRESERVE8 ; Indicate the code here preserve 2: ; 8 byte stack alignment 3: THUMB ; Indicate THUMB code is used 4: AREA.text, CODE, READONLY ; Start of CODE area 5: EXPORT fact 6: ENTRY 7: fact FUNCTION 8: ; expects arg in R0; returns val in R0 2

3 9: PUSH {R1,R2, LR} ; save working registers and LR 10: MOVS R1, R0 ; if nonzero, recursive call... 11: BNE recvcall ; else... 12: MOV R0, #1 13: B return 14: recvcall 15: SUB R0, #1 16: BL fact 17: MUL R0, R0, R1 18: return 19: POP {R1,R2, LR} ; restore saved registers 20: BX LR 21: ENDFUNC 22: END ; End of file The latter is slightly different from the factorial.s of lab 4a as it is not a complete program, just the fact function (compare with lines in lab 4a). Notice line 4: EXPORT fact. This is necessary for the linker to be able to find the textttfact function called by the code in main.c. Open the project in MicroVision, build and load into the ARM target. 1. Place a break-points at line 5 of main.c and single-step in the debugger from there, noting any similarities and differences from the factorial example from lab 4a. 2. Comment in particular on the pattern of subroutine calls and returns. 3. How is parameter passing and returning handled? 2.2 Assembler and MBED At last, and example you can run as a program! You will need a copy of cm0506 ws, the multi-project workspace including the MBED and EBB libraries, that you used last semester. If you do not have this to hand, you can download a fresh copy from the link in week 6 of Semester 1. 3

4 The programs we built using the MBED libraries were compiled in C++ rather then C; C++ manages subroutine calls slightly differently and this has to be taken into account if we want to call our assembly-language fact function from C++. An example is provided by the MicroVision project which you will find in cpp asm MBED.zip (in Lab4Srcs.zip). Unzip cpp asm MBED.zip and move the whole project file hierarchy into the cm0506 ws workspace. The project root, cpp asm MBED should be within cm0506 ws, at the same level as folders mbed nxp lpc408x and ebb. As in the previous exercise, this project cpp asm MBED has two source files in the project. factorial.s is identical to the assemply language source file in the previous exercise, defining the fact function. main.cpp 1: #include <mbed.h> 2: //#include <LM75B.h> 3: #include <display.h> 4: 5: extern "C" int fact(int); //call using to C/Asm conventions 6: 7: Display *screen = Display::theDisplay(); 8: int fact(int x); 9: int x, y; 10: 11: int main() { 12: screen->fillscreen(white); 13: screen->settextcolor(black, WHITE); 14: 15: for (x = 0; x<10; x++) { 16: y = fact(x); 17: screen->setcursor(2,12*x); 18: screen->printf("%d: %d ", x, y); 19: } 20: while(true) 4

5 21: ; 22: return 0; 23: } Note line 5: This line prototypes the fact function defined in factorial.s. Because the source file will be C++-compiled the decoration zcolorredextern C is required to tell the C++ compiler to handle the subroutine call using C (rather than C++) conventions. This is required when subroutine functions are written in C or assembler and called by C++. Excerises 1. Build and load this system into an ARM target and reset the target (no need for the debugger) and report what happens. 2. Make a copy of uapprxsqt.s (see last week s lab: it is in the asm-only folder) and edit it so that defines just the usqt function, not the complete program, and exports it. Move it into the present project s src folder and add it to the build. Edit main.cpp to include some calls to this function - say, to display on the screen values of usqt(x) for 1000 x Report on the result and include listings of your modified main.cpp and uapprxsqt.s. 5