7/23 C Programming in Embedded Systems Computer Science & Engineering Department Arizona State University Tempe, AZ Dr. Yann-Hang Lee (480)

set C Programming in Embedded Systems  Assembly language  dependent of processor architecture  cache control, registers, program status, interrupt  High-level language  memory model  independent of processor architecture (partially true)  Advantages and disadvantages  performance  code size  software development and life cycle

set Manage IO Operations Using C  Access memory-mapped IO – pointers  Example #define REG_READ (a, val) ((val) = *(volatile unsigned char *)(a)) #define REG_WRITE (a, val) (*(volatile unsigned char *)(a) = (val)) #define UART_USR00x4000_0204 #define UART_CR 0x4000_0208 #define UART_RX_EN 1 #define UART_TX_EN (1<<2) char CR_word=0; CR_word |= UART_RX_EN | UART_TX_EN; REG_WRITE (UART_CR, CR_word);

set Platform.h for MXL #ifndef __MCF5213_UART_H__ #define __MCF5213_UART_H__ /* Register read/write macros */ #define MCF_UART0_UMR (*(vuint8 *)(&__IPSBAR[0x000200])) #define MCF_UART0_USR (*(vuint8 *)(&__IPSBAR[0x000204])) #define MCF_UART0_UCSR (*(vuint8 *)(&__IPSBAR[0x000204])) #define MCF_UART0_UCR(*(vuint8 *)(&__IPSBAR[0x000208])) According to Linux C/C++ coding style: “_” Variable not intended to be exposed externally to the user. Internal workings only. Most often used by compiler and library authors. Sometimes used (not preferred) as a suffix to represent a class member variable. “__”Marks an use as extension to ANSI C++. Often not compiler independent. Usually reserved for use by compiler writers.

set Bit Manipulation  Boolean operation  operate on 1 (true) and 0 (false)  (2 || !6 ) && 7 ??  Bitwise operation  operate on individual bit positions within the operands  (2 | ~6 ) & 7 = (0x0002 OR 0xFFF1) AND 0x0007 if (bits & 0x0040) if (bits & (1 <<6)) bits |= (1 <<7)bits &= ~(1<<7) long integer: bits &= ~(1L << 7) OperationBoolean op.Bitwise op. AND&&& OR||| XORunsupported^ NOT!~

set Interface C and Assembly Language  Why combine C and assembly language  performance  C doesn’t handle most architecture features, such as registers, program status, etc.  Develop C and assembly programs and then link them together  at source level – in-line assembly code in C program  at object level – procedure call mwccmcf mwasmcf mwldmcf

set Calling Convention  GCC calling convention  arguments passed in registers and in stack  registers saved by caller and callee (including frame pointer and returning PC)  frame pointer points just below the last argument passed on the stack (the bottom of the frame)  stack pointer points to the first word after the frame saved registers (by callee) dynamic area local variables argument x argument y frame pointer stack pointer saved registers (by caller)

set Coldfire Calling Convention  Passes all parameters on the stack in reverse order.  Push the last argument first  Compact — Passes on even sized boundary for parameters smaller than int (2 for short and char).  Standard — Like compact, but always padded to 4 bytes.  Register — Passes in scratch registers D0 — D2 for integers, A0— A1 for pointers.  Returning  returns an integer value in register D0.  returns a pointer value in register A0.  If it returns a value of any other type,  the caller reserves temporary storage area for that type in the caller's stack and passes a pointer to that area as the last argument.  the called function returns its value in the temporary storage area.

set Register Usage by Coldfire C Compiler  Save code pointer (the value of pc) allows the function corresponding to a stack backtrace structure to be located  Save frame and stack pointers to locate stack frame  Register usage  A0-A1, D0-D2 – scratch registers  A2 through A5 — for pointers  D3 through D7 — for integers and pointers.  A6 – frame pointer  A7 – stack pointer  Caller needs to save D0-D2 and A0-A2 before calling if the values must be preserved  Callee needs to save D3-D7 and A3-A5 if they are used in the procedure

set Stack Usage by Coldfire C Compiler  Allocation of local variable in stack  Save frame and stack pointers to locate stack frame  Entry code link a6, #-framesize ; save and set up fp and sp, movem.l….., -(sp); save registers move.l(.., a6), d0; retrieve parameters ; link instruction: SP – 4 → SP; Ay → (SP); ; SP → Ay; SP + dn → SP  On function exit move.l …., d0; return value movem.l (sp)+,….; restore registers unlka6; restore sp and fp ; unlk instruction -- Ax → SP; (SP) → Ax; ;SP + 4 → SP rts

set Calling Assembly Routine from C  In C program char *srcstr = "First string - source "; char *dststr = "Second string - destination "; strcopy(dststr,srcstr);  Assembly routine.global strcopy strcopy:move.l (4,sp), a0; a0 points to destination string. move.l(8,sp), a1; a1 points to source string. next:move.b (a1)+, d0; Load byte and update address. move.b d0, (a0)+ ; Store byte and update address bne.bnext; Check for zero terminator. rts ; Return..end

set Inline Assembly Code in C Program  A feature provided by C compiler  to specify that a block of code in your file as assembly language  use the “asm” keyword.  compiler will do the insertion and knows the variables and the registers  Example: function level long int b; struct mystruct { long int a; } ; static asm long f(void) // Legal asm qualifier { move.l struct(mystruct.a)(A0),D0 // Accessing a struct. add.l b,D0 // Using a global variable, put // return value in D0. rts // Return from the function: // result = mystruct.a + b }

set Inline Assembly and Access Control Registers  Statement level long square(short a) { long result=0; asm { move.w a,d0 // fetch function argument ‘a’ mulu.w d0,d0 // multiply move.l d0,result // store in local ‘result’ variable } return result; }  Access local and global variables and inline assembly directives  Write to the special purpose registers _mcf5xxx_wr_sr: move.l 4(SP),D0 move.w D0,SR rts _mcf5xxx_wr_vbr: move.l 4(SP),D0.long 0x4e7b0801 /* movec d0,VBR */ nop rts

set Example sumsq int *sum, array[5]; void sumsq (int *sum, int size, int array[]) { int total=0; int i; for ( i = size-1; i < 0; i--) total= total + array[i]^2; *sum=total; } int main() { sumsq(sum, 5, array); while(1);// Idle }

set Example sumsq -- main int *sum, array[5]; ; 19: int main() ; 20: { ; 21: 0x _main: ; main: 0x x4E link a6,#0 0x x4FEFFFF4 lea -12(a7),a7 ; ; 22: sumsq(sum, 5, array); ; 23: ; 0x x41F lea _array,a0 0x E 0x2F move.l a0,8(a7) 0x x7005 moveq #5,d0 0x x2F move.l d0,4(a7) 0x x41F lea _sum,a0 0x E 0x2010 move.l (a0),d0 0x x2E80 move.l d0,(a7) 0x x4EB jsr _sumsq

set Example sumsq -- main ; ; 24: while(1);// Idle ; 25: ; 0x x60FE bra.s *+0 ; 0x x A 0x4E71 nop ; 10: ; 11: void sumsq (int *sum, int size, int array[]) ; 12: { ; 0x _sumsq: ; sumsq: 0x x4E link a6,#0 0x x4FEFFFF4 lea -12(a7),a7 ; 13: int total=0; ; 14: int i; 0x x7000 moveq #0,d0 0x A 0x2D40FFF8 move.l d0,-8(a6)

set Example sumsq ; ; 15: for ( i = size-1; i < 0; i--) ; 0x E 0x202E000C move.l 12(a6),d0 0x x5380 subq.l #1,d0 0x x2D40FFF4 move.l d0,-12(a6) 0x x6030 bra.s *+50 ; 0x a ; ; 16: total= total + array[i]^2; ; 0x A 0x202EFFF4 move.l -12(a6),d0 0x E 0x2D40FFFC move.l d0,-4(a6) 0x x202EFFFC move.l -4(a6),d0 0x x206E0010 movea.l 16(a6),a0 0x A 0x222EFFFC move.l -4(a6),d1 0x E 0x202EFFF8 move.l -8(a6),d0 0x xD0B01C00 add.l (a0,d1.l*4),d0 0x x0A eori.l #0x2,d0 ; '....'

set Example sumsq 0x C 0x2D40FFF8 move.l d0,-8(a6) 0x x202EFFF4 move.l -12(a6),d0 0x x5380 subq.l #1,d0 0x x2D40FFF4 move.l d0,-12(a6) 0x A 0x202EFFF4 move.l -12(a6),d0 0x E 0x4A80 tst.l d0 0x x6DC8 blt.s *-54 ; 0x a ; ; 17: *sum=total; ; 0x x206E0008 movea.l 8(a6),a0 0x x202EFFF8 move.l -8(a6),d0 0x A 0x2080 move.l d0,(a0) ; ; 18: } 0x C 0x4E5E unlk a6 0x E 0x4E75 rts