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Please xplain

Scripture k 

  259           c1_tx_fifo_in_gc++;
   \   0000001C   ....               LDI     R30, LOW(c1_tx_fifo_in_gc)
   \   0000001E   ....               LDI     R31, HIGH(c1_tx_fifo_in_gc)
   \   00000020   ....               LDI     R19, (c1_tx_fifo_in_gc) >> 16
   \   00000022   BF3B               OUT     0x3B, R19
   \   00000024   8100               LD      R16, Z
   \   00000026   9503               INC     R16
   \   00000028   8300               ST      Z, R16
    260                 c1_tx_fifo_in_gc &= (MAX_TX_FIFO - 1);
   \   0000002A   9110....           LDS     R17, LWRD(c1_tx_fifo_in_gc)
   \   0000002E   9310....           STS     LWRD(c1_tx_fifo_in_gc), R17
   \   00000032   E000               LDI     R16, 0
   \   00000034   BF08               OUT     0x38, R16

please explain the above, I am unable to understand the Assembly routines for the above two "C Statements".

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  • 0 in reply to Andy Neil

    Note that (this is C code, not assembler) having an 8-bit index variable for spanning a 2^8 element large FIFO isn't so fun. No fun at all.

    You have an array that can store 0 to 256 elements - which is 257 alternatives.
    You have an index pointer that can store 0 to 255 values - which is 256 alternatives.

    Dropping down to decoding the assembler instructions isn't the first step to do when something doesn't work. It takes understanding to catch a bug, and many times that understanding works better on the C code where a compiler haven't added lots of noise while trying to remap your ideas into efficient sequences of instructions.

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  • 0 in reply to Andy Neil

    Note that (this is C code, not assembler) having an 8-bit index variable for spanning a 2^8 element large FIFO isn't so fun. No fun at all.

    You have an array that can store 0 to 256 elements - which is 257 alternatives.
    You have an index pointer that can store 0 to 255 values - which is 256 alternatives.

    Dropping down to decoding the assembler instructions isn't the first step to do when something doesn't work. It takes understanding to catch a bug, and many times that understanding works better on the C code where a compiler haven't added lots of noise while trying to remap your ideas into efficient sequences of instructions.

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