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extension records follow symbols that define and reference procedure
entry points and global variables. the first extension record, the
symbol extension record, defines the type of the return value or
global variable, and (if a procedure or function) the number of
parameters and the types of the first three parameters. if more
parameter type descriptors are needed, one or more argument extension
records follow, each containing four more descriptors. a check level
of 0 specifies no type checking; no extension records follow. a check
level of 1 or more specifies checking of the return value or global
variable type. a check level of 2 or more specifies checking of the
number of parameters, and a check level of 3 specifies checking the
types of each individual parameter. the linker performs the requested
level of type checking between unsatisfied symbols and local or
universal symbols as it resolves symbol references.
union arg_descriptor \{
struct \{
unsigned int reserved: 3; /* not used */
unsigned int packing: 1; /* reserved */
unsigned int alignment: 4; /* byte alignment */
unsigned int reserved2: 1; /* not used */
unsigned int mode: 3; /* use of symbol */
unsigned int structure: 4; /* structure of symbol */
unsigned int hash: 1; /* set if arg_type is hashed */
int arg_type: 15; /* data type */
\} arg_desc;
unsigned int word;
\};
struct symbol_extension_record \{
unsigned int type: 8; /* always st_sym_ext */
unsigned int max_num_args: 8; /* max # of parameters */
unsigned int min_num_args: 8; /* min # of parameters */
unsigned int num_args: 8; /* actual # of parameters */
union arg_descriptor symbol_desc; /* symbol type desc. */
union arg_descriptor argument_desc[3]; /* first 3 parameters */
\};
struct argument_desc_array \{
unsigned int type: 8; /* always st_arg_ext */
unsigned int reserved: 24; /* not used */
union arg_descriptor argument_desc[4]; /* next 4 parameters */
\};
the values for the alignment, mode, structure, and arg_type (when the
data type is not hashed) fields are given in the following table.
value alignment mode structure arg_type
_______________________________________________________________________
0 byte any any any
1 half-word value parm scalar void
2 word reference parm array signed byte
3 dbl word value-result struct unsigned byte
4 name pointer signed short
5 variable long ptr unsigned short
6 64-byte function return c string signed long
7 procedure pascal string unsigned long
8 long ref parm procedure signed dbl word
9 function unsigned dbl word
10 label short real
11 page real
12 long real
13 short complex
14 complex
15 long complex
16 packed decimal
17 struct/array
for procedure entry points, the parameter relocation bits define the
locations of the formal parameters and the return value. normally,
the first four words of the parameter list are passed in general
registers (r26-r23) instead of on the stack, and the return value is
returned in r29. floating-point parameters in this range are passed
instead in floating-point registers (fr4-fr7) and a floating-point
value is returned in fr4. the parameter relocation bits consist of
five pairs of bits that describe the first four words of the parameter
list and the return value. the leftmost pair of bits describes the
first parameter word, and the rightmost pair of bits describes the
return value. the meanings of these bits are shown in the following
table.
bits | meaning
_____|_____________________________________________________
00 | no parameter or return value
01 | parameter or return value in general register
10 | parameter or return value in floating-point register
11 | double-precision floating-point value
for double-precision floating-point parameters, the odd-numbered
parameter word should be marked 11 and the even-numbered parameter
word should be marked 10. double-precision return values are simply
marked 11.
every procedure call is tagged with a similar set of bits (see
relocation information, below), so that the linker can match each call
with the expectations of the procedure entry point. if the call and
entry point mismatch, the linker creates a stub that relocates the
parameters and return value as appropriate.
relocation information
each initialized subspace defines a range of fixups that apply to the
data in that subspace. a fixup request is associated with every word
that requires relocation or that contains a reference to an
unsatisfied symbol. in relocatable object files created prior to hp-
ux release 3.0 on series 800 systems each fixup request is a five-word
structure describing a code or data word to be patched at link time.
object files created on release 3.0 or later contain variable-length
fixup requests that describe every byte of the subspace. the
version_id field in the file header distinguishes these two formats;
the constant version_id is found in older object files, and the
constant new_version_id is found in newer ones.
in older object files, fixups can compute an expression involving
zero, one, or two symbols and a constant, then extract a field of bits
from that result and deposit those bits in any of several different
formats (corresponding to the precision architecture instruction set).
the fixup_request_index field in the subspace dictionary entry indexes
into the fixup request area defined by the file header, and the
fixup_request_quantity field refers to the number of fixup requests
used for that subspace. the structure of a fixup request is contained
in <reloc.h>.
struct fixup_request_record \{
unsigned int need_data_ref: 1; /* reserved */
unsigned int arg_reloc: 10; /* parameter relocation bits */
unsigned int expression_type: 5; /* how to compute value */
unsigned int exec_level: 2; /* reserved */
unsigned int fixup_format: 6; /* how to deposit bits */
unsigned int fixup_field: 8; /* field to extract */
unsigned int subspace_offset; /* subspace offset of word */
unsigned int symbol_index_one; /* index of first symbol */
unsigned int symbol_index_two; /* index of second symbol */
int fixup_constant; /* constant */
\};
/* values for expression_type */
#define e_one 0 /* symbol1 + constant */
#define e_two 1 /* symbol1 - symbol2 + constant */
#define e_pcrel 2 /* symbol1 - pc + constant */
#define e_con 3 /* constant */
#define e_plabel 7 /* symbol1 + constant */
#define e_abs 18 /* reserved */
/* values for fixup_field (assembler mnemonics shown) */
#define e_fsel 0 /* f': no change */
#define e_lssel 1 /* ls': inverse of rs' */
#define e_rssel 2 /* rs': rightmost 11 bits, signed */
#define e_lsel 3 /* l': leftmost 21 bits */
#define e_rsel 4 /* r': rightmost 11 bits */
#define e_ldsel 5 /* ld': inverse of rd' */
#define e_rdsel 6 /* rd': rightmost 11 bits, filled left with ones */
#define e_lrsel 7 /* lr': l' with rounded" constant */"
#define e_rrsel 8 /* rr': r' with rounded" constant */"
/* values for fixup_format (typical instructions shown) */"
#define i_exp14 0 /* 14-bit immediate (ldw, stw) */
#define i_exp21 1 /* 21-bit immediate (ldil, addil) */
#define i_exp11 2 /* 11-bit immediate (addi, subi) */
#define i_rel17 3 /* 17-bit pc-relative (bl) */
#define i_rel12 4 /* 12 bit pc-relative (combt, combf, etc.) */
#define i_data 5 /* whole word */
#define i_none 6 /* not used */
#define i_abs17 7 /* 17-bit absolute (be, ble) */
#define i_milli 8 /* 17-bit millicode call (ble) */
#define i_break 9 /* reserved (no effect on hp-ux) */
in newer object files, relocation entries consist of a stream of
bytes. the fixup_request_index field in the subspace dictionary entry
is a byte offset into the fixup dictionary defined by the file header,
and the fixup_request_quantity field defines the length of the fixup
request stream, in bytes, for that subspace. the first byte of each
fixup request (the opcode) identifies the request and determines the
length of the request.
in general, the fixup stream is a series of linker instructions that
governs how the linker places data in the a.out file. certain fixup
requests cause the linker to copy one or more bytes from the input
subspace to the output subspace without change, while others direct
the linker to relocate words or resolve external references. still
others direct the linker to insert zeroes in the output subspace or to
leave areas uninitialized without copying any data from the input
subspace, and others describe points in the code without contributing
any new data to the output file.
the include file <reloc.h> defines constants for each major opcode.
many fixup requests use a range of opcodes; only a constant for the
beginning of the range is defined. the meaning of each fixup request
is described below. the opcode ranges and parameters for each fixup
are described in the table further below.
r_no_relocation
copy l bytes with no relocation.
r_zeroes insert l zero bytes into the output subspace.
r_uninit skip l bytes in the output subspace.
r_relocation copy one data word with relocation. the word is
assumed to contain a 32-bit pointer relative to its own
subspace.
r_data_one_symbol
copy one data word with relocation relative to an
external symbol whose symbol index is s.
r_data_plabel copy one data word as a 32-bit procedure label,
referring to the symbol s. the original contents of
the word should be 0 (no static link) or 2 (static link
required).
r_space_ref copy one data word as a space reference. this fixup
request is not currently supported.
r_repeated_init
copy l bytes from the input subspace, replicating the
data to fill m bytes in the output subspace.
r_pcrel_call copy one instruction word with relocation. the word is
assumed to be a pc-relative procedure call instruction
(for example, bl). the target procedure is identified
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