ECE 353 Introduction to Microprocessor Systems

1. ECE 353Introduction to Microprocessor Systems Michael Schulte Week 4

2. Data movement instructions Shifter operands Special cases with PC as destination ARM7TDMI ISA Usage Conditional execution and flags updates Special cases of encoding Data Processing Instructions Memory Addressing Models and Modes Memory Allocation Allocation directives, alignment ARM7TDMI Load/Store Instructions Addressing modes Topics “Complexity is our friend.”

3. MOV Instruction • Syntax • MOV{<cond>}{S} <Rd>, <shifter_operand> • RTL • if (cond is true) • Rd  shifter_operand • if((S==1) AND (Rd==R15)) • CPSR  SPSR • Flags (if S is appended and Rd is not R15) • N, Z, C (C is based on shifter operand)

4. Shifter Operands • Immediate (I=1) • 8-bit value, 4-bit rotate code • Signified by #<number> • Register • No shift • Shifted Register • LSL, LSR, ASR, ROR • Shift count from immediate or register • Rotate Right with Extend • Rotates register right 1 bit through CARRY flag

5. Conditional Execution and Flags • If a condition is used, the instruction will only be executed if the condition is true. • Flags are only updated if S is used • Some instructions (B, BL, CMP, CMN, TST, TEQ, etc.) don’t use S • Using flags and conditions • MOVS R1, R1 • MOV R0, #1 • MOVEQ R0, #0 • MOVMI R0, #-1

6. MOV Special Cases • If R15 is the destination of a MOV instruction and S is appended • MOVS R15, R? • In addition to the move into the PC, the CPSR is loaded from the current exception mode’s SPSR. Flags are not affected. • This behavior is intended to only be used to return from exception modes. • Do not do this in user or system mode – there is no SPSR and the results are unpredictable!

7. Specialized Moves • MVN – move negated • Shifter operand is complemented • Note that this is a 1’s-complement (NOT) • MRS – move CPSR/SPSR to GP register • MSR – move GP register to CPSR/SPSR • SWP – swap • SWPB – swap byte • Swap instructions exchange values between memory and registers in an atomic operation

8. Data Processing Instructions • The data processing instructions all use a very similar structure for operands, including a shifter operand • Mnemonic{cond}{S} Rd, Rn, <shifter_operand> • In general, Rd  Rn operation shifter_operand • The non-destructive instructions will not use a destination register (Rd). • Some instructions reverse the operand order • Since shifts can be part of any MOV or data processing instruction, there are no dedicated shift instructions.

9. Data Processing - Logical • AND – bit-wise AND • BIC – bit clear (bit-wise AND with complement of shifter operand) • EOR – bit-wise exclusive-OR • ORR – bit-wise inclusive-OR • TEQ – test equivalence (non-destructive XOR) • TST – test (non-destructive AND)

10. Data Processing - Arithmetic • ADC – add with carry • ADD – add • CMN – compare negative • CMP – compare • RSB – reverse subtract • RSC – reverse subtract with carry • SBC – subtract with carry • SUB - subtract

11. Multiply-Accumulate • 32x32 multiplies - 32-bit result • MLA – multiply-accumulate • MUL – multiply • 32x32 multiplies - 64-bit result • SMLAL – signed long multiply-accumulate • SMULL – signed long multiply • UMLAL – unsigned long multiply-accumulate • UMULL – unsigned long multiply

12. Memory Addressing Models • Linear Memory Addressing • Instructions can specify the complete address • Segmented Memory Addressing • Instructions do not contain the full address, just part of it (the offset) • The remainder of the address is furnished by a page register or a segment register • There may be multiple segment registers • The full physical address is formed by combining the segment/page register and the offset from the instruction • Advantages / disadvantages

13. Memory Addressing Modes • Direct Addressing • The operand address is encoded into the instruction. • In variable length instructions, the full physical address can usually be encoded. • In fixed length instructions, usually only the least significant part of the address can be encoded • The remainder of the address can be set to 0 (base page addressing) • The remainder of the address can be obtained from a page register or segment register.

14. Memory Addressing Modes (cont) • Register Indirect Addressing • The instruction specifies a register that contains the memory address to access • May also support updating the register as part of the instruction (auto-increment, auto-decrement, etc.) • Memory Indirect Addressing • A memory location (encoded in the instruction) contains the address to transfer to/from

15. Memory Addressing Modes (cont) • Indexed Addressing • The physical address is calculated from a constant starting address (encoded in the instruction) and the contents of a register • Typically used for accessing data in arrays • Base address = array starting address • Register holds (element index × element size) • If byte array, element size = 1 • If halfword array, element size = 2 • If word array, element size = 4 • The processor may do the index * element size calculation automatically

16. Memory Addressing Modes (cont) • Based Addressing • The physical address is calculated from a base address contained in a register, plus a constant offset encoded in the instruction • Typically used for accessing information in data structures. • Register holds starting address of structure. • Offset is distance from the start of the structure to the desired structure element. • Code can then access any instance of the structure just by changing register contents to point to it.

17. Memory Addressing Modes (cont) • PC-Relative Addressing • The address is computed by adding an offset value encoded in the instruction to the current value of the program counter. • In many microprocessors, the PC is not part of the programmer’s model, so PC-relative addressing is considered distinct from indexed or based addressing. • ARM will use PC-relative addressing to implement the appearance of direct addressing. • Distance from the instruction to the label must be known at assembly-time.

18. ARM7TDMI Memory Allocation • Memory operands • Stored in little-endian format • Data allocation directives • DCB, DCW, DCD, SPACE, ALIGN • Identifiers and initializers • Constants vs. variables • Arrays and strings • Setting up a data area • Read-write AREAs are all linked into SRAM • Variable naming

19. ARM7 Memory Addressing Modes • All ARM memory addressing modes use a base register • Can also have a constant offset or use another register for the offset • The second register can also be shifted • The apparent ability to use direct addressing with the ARM will be obtained by using PC-relative addressing • Will look at how ARM accomplishes this later • The same idea will be used for the ADR pseudo-instruction

20. ARM7 Load/Store Instructions • LDR/STR • Load and store a 32-bit register • Does not matter if signed or unsigned • Address should be word-aligned • LDRB/STRB • Load and store an unsigned byte • On load, the value is zero-padded to 32-bits

21. ARM7 Load/Store Addressing • Base register +/- immediate offset • Address = (Rn) +/- offset_12 • Syntax: [Rn, +/-#<offset>] • Base register +/- register offset • Address = (Rn) +/- (Rm) • Syntax: [Rn, +/-Rm] • Base register +/- shifted register offset • Address = (Rn) +/- (shifted Rm) • Shift modes with immediate shift count • LSL, LSR, ASR, ROR, RRX • Syntax: [Rn, +/-Rm, shift_mode #count] • Rn is unaffected by these addressing modes

22. ARM7 Load/Store Addressing (cont) • Pre-indexed • Rn is updated with the calculated address • Syntax: [Rn, +/-#<offset>]! • Syntax: [Rn, +/-Rm]! • Syntax: [Rn, +/-Rm, shift_mode #count]! • Post-indexed • Rn is used as the transfer address. Then, Rn is updated with the calculated address • Syntax: [Rn], +/-#<offset> • Syntax: [Rn], +/-Rm • Syntax: [Rn], +/-Rm, shift_mode #count

23. ARM7 Load/Store Instructions (cont) • LDRSB/LDRSH • Load a signed byte/halfword from memory • Byte/halfword is sign-extended to 32-bits • LDRH/STRH • Load and store an unsigned halfword • On load, the value is zero-padded to 32-bits • The addressing modes are similar to LDR/STR, but are more restricted • Base +/- offset_8 • Base +/- register • Pre-indexed and post-indexed

24. ARM7 Load/Store Multiple • LDM/STM load and store multiple registers to memory in a single instruction • Syntax: LDM{<cond>}<addressing_mode>, <Rn>{!}, <registers> • Addressing mode options • IA – Increment by 4 After transfer • IB – Increment by 4 Before transfer • DA – Decrement by 4 After transfer • DB – Decrement by 4 Before transfer • Register write-back controlled by “!” • Registers are always written/read from memory with lowest register number in the lowest address

25. ARM7 Memory Pseudo-Instructions • Direct addressing (i.e. LDR R0, my_label) • Encoded as LDR R0, [PC, #±offset] • ADR (i.e. ADR R0, my_label) • Encoded as ADD/SUB R0, PC, #number • LDR – Load register • LDR R0, =(expression) • If expression is legal immediate value, encodes as MOV/MVN • Otherwise, allocates a word to store expression in, then loads from that word using MOV with PC-relative addressing • LDR R0, =(label) • Allocates a word to store the label’s address in, then loads that word using MOV with PC-relative addressing

26. In-Class Exercise • Create a source code template with a code area and a data area • Allocate a 100 byte array Bytes in the data area • Create a pointer aBytes to the array in the code area • Declare a halfword variable HwVar in the data area and initialize to 0xAA55 • Create a pointer aHwVar to HwVar in the code area • Use byte transfers to set HwVar to 0xCC33 • Copy HwVar into R0 as unsigned and R1 as signed • Store the element’s index into the 0th, 50th, and 100th elements of the aBytes array • If the 50th element of the array is not zero, exchange the 0th and 100th elements of the array • References MOV instructionLDRB instructionSWAPB instructionConditions

27. Wrapping Up • Reading for next week • Chapter 6.12-6.13 • Quiz #1 will be held Wednesday, 2/27 at 7:15pm in room 2345 EH. • Coverage will be over modules 1 and 2. • Calculators are not permitted. You may have a 3x5 card with handwritten notes. • The instruction set documentation will be provided. • No practice exam – make sure you can complete all of the educational objectives • Homework #2 due this Friday • Discussion section tonight from 6:30 to 8:00 in 3534 EH.

28. Hungarian Notation c signed character uc unsigned character i integer ui unsigned integer si short integer li long integer n an integer number where the actual size is irrelevant f float d double s string of characters sz string of characters, terminated by a null character b an integer or character used as a boolean value by single byte ct an integer being used as a counter or tally p pointer to a structure or general void pointer px pointer to a variable of class x, e.g. pi, pf, pli

29. ARM7 Condition Codes

30. MOV Instruction Reference • Syntax • MOV{<cond>}{S} <Rd>, <shifter_operand> • RTL • if (cond is true) • Rd  shifter_operand • if((S==1) AND (Rd==R15)) • CPSR  SPSR • Flags (if S used and Rd is not R15) • N, Z, C (C is based on shifter operand)

31. LDRB Instruction Reference • Syntax • LDRB{<cond>} <Rd>, <addressing_mode> • RTL • if (cond is true) • Rd[7:0]  memory[memory_address] • Rd[31:8]  0 • if(writeback) • Rn  end_address • Flags • None

32. SWPB Instruction Reference • Syntax • SWP{<cond>}B <Rd>, <Rm>, [<Rn>] • RTL • if (cond is true) • temp  (Rn) • (Rn)  Rm • Rd  temp • Flags are not affected

33. Team ConcepTest • What are the primary functions of an assembler? What sorts of errors might it detect? • What are the primary functions of a linker? What sorts of errors might it detect?

34. One Minute Paper • How do you feel that your homework team is functioning? • Would you say that it is hindering, improving, or having no effect on your learning?