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The NS7520

The NS7520. NET+ARM. ARM7TDMI Programmer’s Model. Processor Operating States.

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The NS7520

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  1. The NS7520

  2. NET+ARM ARM7TDMI Programmer’s Model

  3. Processor Operating States • The ARM7TDMI can be in one of two Processor Operating States:- ARM state which executes 32-bit, word-aligned ARM instructions- Thumb state which operates with 16-bit, halfword-aligned Thumb instructions.Note: Transition between these two states does notaffect the operating mode (user, supervisor, …) or the contents of the registers

  4. Switching between OP States • Entering Thumb state:- Execute a BX instruction with the state bit (bit 0) set in the operand register- Transition also occurs automatically when returning from an exception if this was entered with the processor in Thumb state • Entering ARM state- Execute a BX instruction with the state bit clear in the operand register- Also occurs on the processor taking an exception (IRQ, FIQ, RESET, UNDEF, ABORT, SWI, etc.) BX = Branch and Exchange (Operating State)

  5. Memory Formats • ARM7TDMI views memory as a linear collection of bytes numbered upwards from zero. • Bytes 0 to 3 hold the first word, bytes 4 to 7 the second and so on. • The ARM7TDMI can treat words in memory as being stored either in - Big Endian format (also known as Motorola format)- Little Endian format (also known as Intel format)

  6. Big Endian Format Higher Address Word Address 31 24 23 16 15 8 7 0 8 9 10 11 8 4 5 6 7 4 0 1 2 3 0 Lower Address • Most Significant Byte is at lowest address • Word is addressed by byte address of most significant byte

  7. Little Endian Format Higher Address Word Address 31 24 23 16 15 8 7 0 11 10 9 8 8 7 6 5 4 4 3 2 1 0 0 Lower Address • Least Significant Byte is at lowest address • Word is addressed by byte address of least significant byte

  8. Instruction Length and Data Types • ARM7TDMI Instruction Length is- 32 bits in ARM state- 16 bit in Thumb state • ARM7TDMI supported datatypes are:- byte (8-bit)- halfword (16-bit)- word (32-bit) • Words must be aligned to four-byte boundaries and halfwords at two-byte boundaries

  9. ARM7TDMI Registers • ARM7TDMI has a total of 37 registers- 31 general purpose 32-bit registers- 6 status registers • Not all registers can be seen at once! • The processor state and operating mode dictate which registers are available to the programmer

  10. ARM State Register Set • 16 general registers and one or two status registers are visible at one time • Mode-specific banked registers are switched inin privileged, non-User modes • Dedicated registers are:- R14 receives a copy of R15 when a Branch with Link instruction (BL) is executed.- R15 holds the Program Counter- R16 is the CPSR (Current Program Status Register) This contains the condition code flags and the current mode bits.

  11. ARM State Register Set System & User FIQ Supervisor Abort IRQ Undefined R0 R0 R0 R0 R0 R0 R1 R1 R1 R1 R1 R1 R2 R2 R2 R2 R2 R2 R3 R3 R3 R3 R3 R3 R4 R4 R4 R4 R4 R4 R5 R5 R5 R5 R5 R5 R6 R6 R6 R6 R6 R6 R7 R7 R7 R7 R7 R7 R8 R8_fiq R8 R8 R8 R8 R9 R9_fiq R9 R9 R9 R9 R10 R10_fiq R10 R10 R10 R10 R11 R11_fiq R11 R11 R11 R11 R12 R12_fiq R12 R12 R12 R12 SP SP_fiq SP_svc SP_abt SP_irq SP_und LR LR_fiq LR_svc LR_abt LR_irq LR_und PC PC PC PC PC PC CPSR CPSR CPSR CPSR CPSR CPSR SPSR_fiq SPSR_svc SPSR_abt SPSR_irq SPSR_und Banked Registers

  12. Thumb State Register Set System & User FIQ Supervisor Abort IRQ Undefined R0 R0 R0 R0 R0 R0 R1 R1 R1 R1 R1 R1 R2 R2 R2 R2 R2 R2 R3 R3 R3 R3 R3 R3 R4 R4 R4 R4 R4 R4 R5 R5 R5 R5 R5 R5 R6 R6 R6 R6 R6 R6 R7 R7 R7 R7 R7 R7 SP SP_fiq SP_svc SP_abt SP_irq SP_und LR LR_fiq LR_svc LR_abt LR_irq LR_und PC PC PC PC PC PC CPSR CPSR CPSR CPSR CPSR CPSR SPSR_fiq SPSR_svc SPSR_abt SPSR_irq SPSR_und Banked Registers

  13. State Switching Thumb State ARM State R0 R0 R1 R1 R2 R2 R3 R3 R4 R4 R5 R5 R6 R6 R7 R7 R8 R9 R10 R11 R12 Stack Pointer (SP) Stack Pointer (SP) Link Register (LR) Link Register (LR) Program Counter (PC) Program Counter (PC) CPSR CPSR SPSR SPSR

  14. Accessing R8..R12 in Thumb state • R8..R12 are not part of the standard Thumb register set. • There is limited access to R8..R12 in Thumb state, using special variants of the MOV instruction. • R8..R12 can be compared against or added to R0..R7 using CMP and ADD instructions • Conclusion:R8..R12 may have changed when switching back to ARM state.

  15. The Program Status Registers • The ARM7TDMI contains a Current Program Status Register (CPSR), plus five Saved Program Status Registers (SPSR). These- hold Information about the most recently performed ALU operation- control the enabling and disabling of Interrupts- set the Processor operating mode Condition Code reserved Control Bits 31 30 29 28 27 26 8 7 6 5 4 3 2 1 0 N Z C V . . . I F T M4 M3 M2 M1 M0 Overflow Carry/Boorow Zero Negative Mode State FIQ disable IRQ disable

  16. Condition Codes • N, Z, C and V bits are the condition code flags.These may be changed as a result of arithmetic and logical operations and may be tested to determine whether an instruction should be executed. • In ARM state a l l instructions may be executed conditionally • In Thumb state, only the Branch instruction is capable of conditional execution

  17. Control Bits • The T-bitReflects the operating state and is set when in Thumb • The Interrupt disable bitsAre used to mask (disable) the IRQ and FIQ interrupts • The mode bitsM4..M0 determine the operating mode. Not all combinations define a valid operating mode – Illegal values programmed into the mode bits will cause the processor enter an unrecoverable state. If this happens, a reset should be applied. • Reserved bitsShould never be altered.

  18. Operating Mode Summary

  19. Exceptions (1) • Actions on entering an exception- Save return address in the appropriate LR Return address is either current PC + 4 or current PC +8 (depending on the exception)- Copy the CPSR into the appropriate SPSR- Force the CPSR mode bits to a value which depends on the exception- Load PC to fetch next instruction from the relevant exception vectorNote: If the processor is in Thumb state when an exception occurs, it will automatically switch into ARM state when the PC is loaded with the exception vector address.

  20. ARM State Register Set System & User FIQ Supervisor Abort IRQ Undefined R0 R0 R0 R0 R0 R0 R1 R1 R1 R1 R1 R1 R2 R2 R2 R2 R2 R2 R3 R3 R3 R3 R3 R3 R4 R4 R4 R4 R4 R4 R5 R5 R5 R5 R5 R5 R6 R6 R6 R6 R6 R6 R7 R7 R7 R7 R7 R7 R8 R8_fiq R8 R8 R8 R8 R9 R9_fiq R9 R9 R9 R9 R10 R10_fiq R10 R10 R10 R10 R11 R11_fiq R11 R11 R11 R11 R12 R12_fiq R12 R12 R12 R12 SP SP_fiq SP_svc SP_abt SP_irq SP_und LR LR_fiq LR_svc LR_abt Saved PC LR_irq LR_und PC PC PC PC PC PC PC PC PC CPSR CPSR CPSR CPSR CPSR CPSR CPSR CPSR CPSR SPSR_fiq SPSR_svc SPSR_abt SPSR_irq Saved CPSR SPSR_und Banked Registers

  21. Exceptions (2) • Actions when leaving an exception- Load PC, minus an offset where appropriate, from LR (offset will vary depending on the exception)- Copy the SPSR back into the CPSRNote: An explicit switch back to Thumb state is not required, since restoring the CPSR from the SPSR automatically sets the T-bit to the value it held immediately prior to the exception.

  22. Exception entry/exit Summary

  23. Exception Vectors and Priority Note: Undefined Instruction and Software Interrupt are mutually exclusive, since they each correspond to particular (non-overlapping) decodings of the current instruction.

  24. Interrupt Latencies TD = Tsyncmax + Tldm + Tfiq Tsyncmax is 5 clock cycles Tldm is the time for the longest instruction to complete (which is the LDM instruction) and is 23 cycles Tfiq is the time for the FIQ entry and is 2 clock cycles @44 MHz this will give a total of 30 clocks (less than 1us) for the processor to execute the FIQ Instruction at address 0x1c.

  25. Q & A

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