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Embedded Systems Programming Basics

Understand condition codes and data transfer operations in embedded systems programming. Learn how to manipulate data in memory and communicate with peripherals efficiently using various instructions and addressing modes.

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Embedded Systems Programming Basics

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  1. Chapter 3

  2. Condition Codes • N (Negative Flag): • Set if the MSB of a result is set • Clear otherwise • Please note that the MSB depends upon the type of operation involved.

  3. Condition Codes • Z: Zero Flag • Set if the result of an operation is zero. • Clear otherwise.

  4. Condition Codes • V: Overflow • Set when the result represents the possibility of the sign change. • Clear otherwise. • Example: • MOVE.B #$77,D0 V=0 • ADDI.B #30,D0 V=1 • ADDI.B #9,D0 V=0 • SUBI.B #40,D0 V=1 • SUBI.B #4,D0 V=0

  5. Condition Codes • C: Carry Flag • Set is the operation of an Addition result in a carry out • Clear otherwise.

  6. Condition Codes • X: Extend Flag: • Is not set directly by the results of the operations. • Many instructions have no effect on the carry flag • Those set this does in the same way as the carry flag.

  7. DATA TRANSFER GROUPWriting Data into Memory • D0: 55556666 • D1: 77778888 • D2: 9999AAAA • D3: BBBBCCCC • TAB1: refers to the data area in memory that is located at the address 0030B8. • MOVEM.W D0-D3, TAB1….. • MOVEM.L D0-D3, TAB1

  8. Writing Data into Memory • Can you think about any other way doing the same operation • Hint: • Address register Indirect • Address register Indirect with Predecrement

  9. Reading Data From Memory • MOVEM.L #data, D0/D1; • Can you think of other ways of doing the same operation

  10. Moving Peripheral Data • For communication with all the peripherals • Note: Only one-half of the system bus is used for the peripheral data • MOVEP • Accesses two locations at a time • Only long-word and word data size is allowed • No condition codes are affected • Only register indirect with displacement addressing is allowed.

  11. Moving Peripheral Data • MOVEP.W D2,0(A1) • D2 contains: 12345678 • A1 contains: 00045000 • MOVEP.W 0(A1), D2

  12. Quick Move • MOVEQ • Used for moving only 8-bits of data • The data is moved to one of the eight data registers. • Before the data is moved the data is sign extended to 32bits. • All the condition codes else than X are affected • X remains unchanged • So why do we use MOVEQ and not MOVE? • MOVEQ #B7, D4?

  13. Push Effective Address • Where does the data go or say where we want to push the data? • No condition codes are affected • The effective address of the operand is computed and translated into 32-bit value before being pushed onto processor’s stack. • PEA $40,(A5)? • A5: 00003060

  14. SWAP • For swapping register halves • The upper and the lower word of the data register are swapped. • No condition code is affected • D5: 3CFF9100 • SWAP D5

  15. Exchange • Any of the 16 data and address registers may be exchanged • EXG D3, D5

  16. Load Effective Address • It is used to load 24 bit address into address register • Whenever the addressing mode is used ro compute the address of an operand, the result is called the effective address. • No condition codes are affected • The value is sign extended before being loaded • LEA $8500, A1 • LEA $10(PC), A1

  17. LINK • Link and allocate • Used for allocating the stack space and implementing linked lists • The specific address register is pushed onto stack • SP is copied into the specified address register • The signed 16-bit displacement is added to the SP • No condition codes are affected • A0: 00006200 A2: 0000FFC4 • LINK A0,#$FFF0; • In any stack operation the SP is first decremented • Since the address value is being pushed the SP is decremented by 4 • Next A0 is loaded with the new value of the SP. • Then the signed displacement is added to the SP.

  18. UNLINK • Opposite of Link operation • A2: 0009FFB4 • UNLK A2 • What will be value of A2 register and the final stack pointer

  19. Arithmetic GroupADD • To add binary numbers • Can be used for 8-bit, 16-bit, 32-bit data. • All the condition codes are affected as per the result of the operation

  20. ADD • D2: 12345678 • D3: 5F02C332 • ADD.B D2,D3

  21. Add Address • ADDA • To add the data to a address register • All the addressing modes may be used. • Only word and long word data size is possible • The condition codes are not affected

  22. Add Address • ADDA.W A0, A3 • A0:CE001A2B • A3:00140300

  23. Add Immediate • Byte, word and long-word values may be added to the destination operand • PC relative addressing not allowed • The destination can not be the address register • All the condition codes are affected

  24. Add Immediate • ADDI.B #$10,D2 • ADDI.W #$10,D2 • D2: 250C30F7

  25. Other Addition Operations • ADDQ • Quick add • Similar to immediate add else than the fact that the data is in the range of 0 to 8 • Adding the immediate data to a address register affects the complete 32bit register. • ADDX • Only two forms of addressing allowed • Data register to data register • Memory to memory with address register indirect with predecrement • The content of the X flag is included in the addition operation • All the data sizes may be used • All the CC are affected

  26. Add with Extend • ADDX.B D2, D3 • ADDX.W -(A0), -(A1)

  27. Clear • Clear an operand • This instruction writes 0s into the location specified • All the three data sizes may be used • All the condition codes may be affected

  28. Clear Example • CLR.B D0 • CLR.W A4 • CLR.L ARRAY

  29. Compare • Compare the data with the data register and set the condition codes accordingly • All the three data sizes may be used • All the condition codes except X are affected • CMP.W #$29AF,D6 • D6:485C29AF

  30. Other Compare Operations • CMPA.L A2, A3 • CMPI.W #5,(A3) • PC relative addressing may not be used. • CMPM • The source and the destination operands must be specific using address register indirect with post-increment. • All the data Sizes may be used • All the CC except X are affected • Two data areas 10 bytes each reside in the memory. Address register A1 and A2 points to the 1st byte in each data table. How can be do this?

  31. Divide • DIVS • Signed 32 bit number to be divided by a signed 16 bit number. • The destination operand must be a data register. • Only word size may be used • After execution lower 16bit contains the quotients and upper 16 bit contains the remainder. • The sign of the remainder is always the same as the sign of the sign of the dividend. • All the CC except X are affected.

  32. Divide • D2: FFFFFC18 • D3: 000186A0 • D4: 000001F4 • D5: 000009C4 • DIVS D2, D3 • DIVS D4,D5

  33. Divide • DIVU • D4: 0000019A D5: 0007A120 • DIVU D4, D5

  34. Extend • EXT • Used to extend the sign of the data register into the remaining upper bits of the register. • Word and long word operations are allowed • All CC except X are affected • EXT.W D3 • EXT.L D3 • D3: 000000C6

  35. Multiply • MULS • 16 by 16 signed multiplication • Data registers are used as the destination and the source • The long word of the data register us used during the operation • All CC expect X are affected

  36. Multiply • MULS D4, D5 • D4: 0000FFF0 D5:0000FFF6 • MULU • MULU D4, D5 • D4: 0000000A D5:00000064

  37. Negate • This is used to generate 2’s complement of a number by subtracting from zeros. • All the data sizes may be used. • All the condition codes are affected. • NEG.B D2 • D2: 052055C6 • NEGX

  38. Subtract • Follows the same pattern as ADD instruction

  39. Absolute Short Addressing • How to know that we are trying to move a data directly or a data which is stored at some memory location • To access the data in two specific ranges of memory each of which are 32KB. • Starting address of one of them is 000000 through 007FFF. • Starting address of the second location is 008000 through FFFFFF

  40. Absolute Short Addressing • MOVE.B $3C00,D1 • MOVE.W $9AE0,D2

  41. Absolute long Addressing • Here no sign extension takes place. • The full 24-bit address has to be provided. • Any address within the range 000000 to FFFFFF may be used with this instruction. • Thus two additional words are used after the op-code by the compiler. • Example: • MOVE.B $2E000,D0

  42. Program Counter With Displacement • This uses the 68000’s PC along with the signed 16-bit displacement. • The signed displacement allows accessing the memory 32768 location behind and 32767 location forward in the program. • This is used to implement the branch instructions and also for referencing the memory. • The required assembler syntax are: d(PC) and (d,PC)

  43. Immediate Data • For loading some immediate value into some register. • # sign must be included for the immediate data. • Example: • MOVE.B #$3A,D5

  44. Quick Immediate Data • This is the same as the immediate data movement • The only difference is that only byte value is suppose to be moved. • MOVEQ #$2C,D3

  45. Implied Addressing • This makes the reference to the processor register within its mnemonics. • Example: • MOVE CCR,CODES • ANDI #$27,SR • TRAPV

  46. Shift and Rotate Group • ASL Arithmetic shift left • ASR Arithmetic shift right • LSL Logical shift left • LSR Logical shift Right • ROL Rotate Left • ROR Rotate Right • ROXL Rotate Left with Extend • ROXR Rotate Right with Extend

  47. Shift and Rotate Group • All the instruction may use byte, word and long-word data sizes. • Only data register and memory words are used as operand. • The count must be supplied so to by how many bits we want to shift or rotate. • When memory location is destination then only 1bit may be shifted at a time and the data size is limited to word operation in this case. • When shift or rotate count is greater than 8-bits then it must be placed in the data register.

  48. Shift and Rotate Group • ASL.B #4,D2 • LSR.W #6,D1 • ROR.L #3,D5 • ROXR.B #5,D4 • ASL.L D2,D3 • ROR.W (A0) • FLAGS: N, Z and C flags are affected by all the eight instructions in the same way. • V flag is always cleared, except when ASL or ASR is used. In this case it will be set if the sign change occur during the operation. • Six of the eight instructions also affect the X bit.

  49. Shift and Rotate Group • ASL (Arithmetic Shift Left) • A zero is shifted into LSB while the bits are moved to left • The bits shifted out of the MSB replaces the X and C flag.

  50. Shift and Rotate Group • ASL (Arithmetic Shift Left) • All the bits are shifted to the right • The bit which leaves the LSB position replaces the X and C flag. • The MSB is shifted back into itself. • This is important when we want to preserve the sign of some register.

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