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TMS320F28335: compiler does not reserve space for string in struct

Jinhui Zhang
Prodigy 70 points
Part Number: TMS320F28335

I am using an example in the assembly tools document SPRU513U on page 138, where in the struct there is a string as a member. The listing file generated by the example has a line of opcode that is confusing.

TMS320C2000 Assembler PC v18.12.3 Mon Feb 1 08:39:59 2021

Copyright (c) 1996-2018 Texas Instruments Incorporated
directive_struct4.asm PAGE 1

1 BIT_REC .struct ; stag
2 0000 STREAM .string 64
3 0040 BIT7 .field 7 ; bits1 = 64
4 0040 BIT9 .field 9 ; bits2 = 64
5 0041 BIT10 .field 10 ; bits3 = 65
6 0042 X_INT .int ; x_int = 67
7 0043 BIT_LEN .endstruct ; length = 68
8 BITS .tag BIT_REC
9 00000000 8100- ADD ACC, @BITS.BIT7 ; move into acc
10 00000001 3E00 AND ACC, #007Fh ; mask off garbage bits
00000002 007F
11 00000000 BITS .usect ".ebss", BIT_REC

In line 9, the opcode it generates is 8100 while I think it should be 8140 as BITS.BIT7 is at address 0040.
If I replace the .string element with any other types, BITS.BIT7 would be the right address.

Please shed some light on this. Thanks.

  • 0
    TI__Guru**** 244030 points

    Jinhui Zhang said:
    In line 9, the opcode it generates is 8100 while I think it should be 8140

    I understand why you think that is the case.  But you are overlooking the presence of the relocation, and how it affects the listing.

    Take a closer look at line 9 ...

    Jinhui Zhang said:
    9 00000000 8100- ADD ACC, @BITS.BIT7 ; move into acc

    Notice the - character right after the opcode 8100.  That means this instruction has a relocation associated with it, and this relocation is relative to the .ebss section.  For more details, please search the C28x assembly tools manual for the sub-chapter titled Field 3: Object Code.  

    To see more details about this relocation, use the object file display utility ofd2000.  

    % ofd2000 -v --obj_display=none,relocs directive_struct4.obj

OBJECT FILE:  directive_struct4.obj

 Relocation Table 1/1 for Section 2 (".text")

    <0> R_PARTLS6 - Symbol-Relative Relocation
       Offset:        0x00000000  Symbol:  .ebss (10)
       Displacement:  0x0040      (null)   (null)

    Note the relocation stores the offset of 0x0040 words.

    The object file display utility ofd2000 is documented in the C28x assembly tools manual.

    Thanks and regards,

    -George

  • 0 in reply to George Mock
    Prodigy 70 points

    thanks George. It's clear now. I knew it was my ignorance but just don't know where. Using -v makes displacement show up and it explains it. Thanks again.