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Examples:

hold .word 1 ; reserve one word of storage and initialize to 1 count .word 0 ; reserve one word of storage and initialize to 0 . . . . ld *(count),B ; sets B = 0 (assuming memory was not changed) st T,*(hold) ; sets (storage location at address hold) = T

Indirect addressing: smem, xmem, ymem

Indirect addressing on the TMS320C54x always uses the auxiliary registers AR0 through AR7 and comes in two basic flavors. These are easily recognized from the assembly language syntaxdescriptions as either Smem or Xmem/Ymem .

Smem

In Smem indirect addressing, only one indirect address is used in the instruction and a number ofvariations is possible (see the table on page 5-13 of the CPU and Peripherals reference). An asterisk is always used, which signifies indirect addressing. Any of theregisters AR0 - AR7 may be used, with optional modifications: automatic post-decrement byone, pre- and post-increment by one, post-increment and post-decrement by n ( n being stored in AR0 ), and more, including many options for circular addressing (which automaticallyimplements circular buffers) and bit-reversed addressing (which is useful for FFTs).

Xmem/ymem

Xmem/Ymem indirect addressing is generally used in instructions that need two different indirect addresses,although there are a few instances where an Xmem by itself is specified in order to save bits in the opcode for other options. In Xmem/Ymem indirect addressing, fewer bits are used to encode the option modifiers in the opcode; hence,fewer options are available: post-increment by one, post-decrement by one, and post-increment by AR0 with circular addressing.

Examples:

stl B,*AR6 ; sets (contents of location pointed to by AR6) = low word of B stl B,*AR6+0% ; sets (contents of location pointed to by AR6) = low word of B, ; then increments AR6 with circular addressing mar *+AR3(-6) ; decrements AR3 by 6 (increment by -6)

The mar (modify address register) instruction is unusual in the sense that it takes an Smem operand but does nothing with the data pointed to by the ARx register. Its purpose is to perform any of the allowed register modifications discussed abovewithout having to do anything else. This is often handy when you are using an Xmem/Ymem -type instruction but need to do an ARx modification that is only allowed with an Smem -type operand.

Summary

The ld instruction is illustrative of the many possible addressing modes which can be selected with theproper choice of assembly language syntax:

ld #0,A ; immediate data: sets A = 0 ld 0,A ; DP-referenced direct: sets A = (contents of the address DP + 0) ld mydata,A ; DP-referenced direct: sets A = (contents of the address ; DP + lower seven bits of mydata) ld #mydata,A ; immediate data: sets A = 16 bit address mydata ld *(mydata),A ; *(lk) direct: sets A = (contents of the 16 bit address mydata) ld B,A ; accumulator: sets A = B ld *AR1+,A ; indirect: sets A = (contents of address pointed to by AR1), ; and afterwards increments AR1 by one ldm AR2,A ; memory-mapped register: sets A = AR2

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Source:  OpenStax, Ece 320 - spring 2003. OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10096/1.2
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