Instruction Sets

78 CHAPTER 2 Instruction SetsThe block repeat start and end registers RSA0L and REA0L keep track of the startand end points of the block.The block repeat register 1 BRC1 and block repeat save register 1 BRS1 are usedto repeat blocks of instructions. There are two repeat start address registers RSA0and RSA1. Each is divided into low and high parts: RSA0L and RSA0H, for example.Four temporary registers T0,T1,T2, and T3 are used for various calculations.Two transition register TRN0 and TRN1 are used for compare-and-extractextremuminstructions. These instructions are used to implement the Viterbialgorithm.Several registers are used for addressing modes. The memory data page startaddress registers DP and DPH are used as the base address for data accesses. Similarly,the peripheral data page start address register PDP is used as a base for I/O addresses.Several registers control interrupts.The interrupt mask registers 0 and 1,namedIER0 and IER1, determine what interrupts will be recognized. The interrupt flagregisters 0 and 1, named IFR0 and IFR1, keep track of currently pending interrupts.Two other registers,DBIER0 and DBIER1,are used for debugging.Two registers,theinterrupt vector register DSP ( IVPD) and interrupt vector register host ( IVPH) areused as the base address for the interrupt vector table.The C55x registers are summarized in Figure 2.17.The C55x supports a 24-bit address space,providing 16 MB of memory as shownin Figure 2.18. Data,program,and I/O accesses are all mapped to the same physicalmemory. But these three spaces are addressed in different ways.The program spaceis byte-addressable, so an instruction reference is 24-bit long. Data space is wordaddressable,soa data address is 23 bits. (Its least-significant bit is set to 0.)The dataspace is also divided into 128 pages of 64K words each.The I/O space is 64K wordswide, so an I/O address is 16 bits. The situation is summarized in Figure 2.19.Not all implementations of the C55x may provide all 16 MB of memory on chip.The C5510,for example,provides 352 KB of on-chip memory.The remainder of thememory space is provided by separate memory chips connected to the DSP.The first 96 words of data page 0 are reserved for the memory-mapped registers.Since the program space is byte-addressable,unlike the word-addressable data space,the first 192 words of the program space are reserved for those same registers.2.3.2 Addressing ModesThe C55x has three addressing modes:■ Absolute addressing supplies an address in the instruction.■Direct addressing supplies an offset.■ Indirect addressing uses a register as a pointer.Absolute addresses may be any of three different types:■ A k16 absolute address is a 16-bit value that is combined with the DPH registerto form a 23-bit address.