HP 3000 Manuals HP 3000 Manuals
Changed Features Some features are implemented differently in HP FORTRAN 77/V and HP FORTRAN 77/iX. These are explained below. Word Size The main difference affecting conversion from HP FORTRAN 77/V to HP FORTRAN 77/iX is word size. The machine word size on computers running the MPE V operating system is 16 bits, while that on computers running MPE/iX is 32 bits. Because of the 32-bit word size, programs declaring data items to be INTEGER*2 run slower on HP FORTRAN 77/iX than programs declaring them to be INTEGER*4. (A feature that improves performance in HP FORTRAN 77/V may decrease it in HP FORTRAN 77/iX.)
NOTE Data items declared as INTEGER default to 32 bits in both FORTRAN 77/V and HP FORTRAN 77/iX (unless the SHORT compiler directive is used, which makes INTEGER data items 16 bits).
Floating-Point Data HP FORTRAN 77/iX represents data items of types REAL, DOUBLE PRECISION, COMPLEX, and DOUBLE COMPLEX with the IEEE floating-point standard. HP FORTRAN 77/V uses a proprietary HP 3000 floating-point format for these items. Uninitialized Variables The MPE/iX Link Editor does not initialize stack space for all variables as the Segmenter does on MPE V. Therefore, uninitialized variables that do not cause problems on MPE V may cause programs to abort on MPE/iX based systems. To avoid this, ensure that all variables are properly initialized. Alignment The default alignment of data items larger than 16 bits is different in HP FORTRAN 77/iX and HP FORTRAN 77/V. The term alignment here refers to a data item's position in memory relative to the adjacent word boundaries. For example, in both versions of HP FORTRAN 77 an INTEGER*4 variable is aligned by default on a word boundary in both versions of HP FORTRAN 77. Therefore, the alignment is on a 16-bit boundary in HP FORTRAN 77/V and on a 32-bit boundary in HP FORTRAN 77/iX (see Figure 8-1 ). The compiler accomplishes the alignment by leaving "holes" (unallocated memory locations) where necessary in the allocated memory space of a program. Consider the following declarations: INTEGER*4 i4 CHARACTER*5 ch5 INTEGER*2 i2 REAL*8 d8 Figure 8-1 compares how these data items might be allocated in memory in HP FORTRAN 77/V and in HP FORTRAN 77/iX:Figure 8-1. Data Alignment Comparison In Figure 8-1 , both versions of HP FORTRAN 77 assign the first hole in the same position after the character variable, because the following data item, which is declared as an INTEGER*2, is 16-bit aligned in both. HP FORTRAN 77/iX assigns a second hole to align the double precision variable D8 on a double-word (64-bit) boundary. In HP FORTRAN 77/V, on the other hand, double precision items need only be word (16-bit) aligned. In the absence of an EQUIVALENCE statement, the compiler leaves these holes and allocates memory to align each variable optimally. An EQUIVALENCE statement can attempt to force illegal alignments. For example: CHARACTER ch(12) INTEGER*4 i, j EQUIVALENCE (ch(1), i), (ch(6), j) is illegal in both FORTRAN 77/V and HP FORTRAN 77/iX, because it attempts to align j on an 8-bit boundary (by forcing an equivalence between j and the sixth element of ch). However, the following: CHARACTER ch(12) INTEGER*4 i, j EQUIVALENCE (ch(1), i), (ch(7), j) is legal in HP FORTRAN 77/V, but not in HP FORTRAN 77/iX, because it forces j to be aligned on a 16-bit boundary (by forcing an equivalence between j and the seventh element of ch), instead of on the 32-bit boundary dictated by HP FORTRAN 77/iX. The compiler responds to illegal alignment requests with the error message ILLEGAL OR INCONSISTENT EQUIVALENCE STATEMENT However, HP FORTRAN 77/iX allows the above use of EQUIVALENCE if the HP3000_16 ON or HP3000_16 ALIGNMENT compiler directive is used. The change in alignment requirements affects items that are equivalenced differently from MPE/iX defaults. The following table shows all nine data types and their alignment requirements in HP FORTRAN 77/V and HP FORTRAN 77/iX: Table 8-1. Data Alignment on HP FORTRAN 77/V and HP FORTRAN 77/iX ----------------------------------------------------------------------------- | | | | | Data Type | HP FORTRAN 77/V | HP FORTRAN 77/iX | | | | | ----------------------------------------------------------------------------- | | | | | REAL*8 | 16 bits | 64 bits | | | | | ----------------------------------------------------------------------------- | | | | | COMPLEX*16 | 16 bits | 64 bits | | | | | ----------------------------------------------------------------------------- | | | | | COMPLEX*8 | 16 bits | 32 bits | | | | | ----------------------------------------------------------------------------- | | | | | REAL*4 | 16 bits | 32 bits | | | | | ----------------------------------------------------------------------------- | | | | | INTEGER*4 | 16 bits | 32 bits | | | | | ----------------------------------------------------------------------------- | | | | | LOGICAL*4 | 16 bits | 32 bits | | | | | ----------------------------------------------------------------------------- | | | | | INTEGER*2 | 16 bits | 16 bits | | | | | ----------------------------------------------------------------------------- | | | | | LOGICAL*2 | 16 bits | 16 bits | | | | | ----------------------------------------------------------------------------- | | | | | Character | 8 bits | 8 bits | | | | | ----------------------------------------------------------------------------- | | | | | Common | 16 bits | 64 bits | | | | | ----------------------------------------------------------------------------- Loading or storing data items that are not aligned on MPE/iX default boundaries as shown in this table requires the compiler to generate special code sequences to adjust for the differences in alignment. The HP3000_16 ALIGNMENT directive turns on generation of these sequences and allows equivalences that would otherwise be illegal in MPE/iX. Common Blocks Alignment also becomes an issue when common blocks are defined differently between program units. The compiler is required to allocate the items in a common block in the order of their occurrence in the COMMON statement. Holes are inserted in the allocated common area to align each variable according to its declared data type. If the type declarations for the variables in a given common block vary between program units, the holes may vary in size, according to the data types of the surrounding variables. Therefore, what was a size match for two definitions on MPE V may match a variable with a hole on MPE/iX. If the type definitions vary between the program units of a common block, the program may not be portable between machines with different alignment rules. Fortunately, this is not a common practice. Here is an example showing alignment differences for common block variables in two program units. The declarations SUBROUTINE sub1 COMMON /blk1/ch,int4 CHARACTER ch INTEGER*4 int4 produce the allocations shown below on MPE V and MPE/iX:The declarations SUBROUTINE sub2 COMMON /blk1/ch,int2 CHARACTER ch INTEGER*2 int2 produce the allocations shown below on MPE V and MPE/iX:In the allocations above, note that for the common block blk1, int2 overlays int4 on MPE V but on MPE/iX int2 overlays two unallocated bytes. Using the HP3000_16 ALIGNMENT option would resolve this alignment problem. SYSTEM INTRINSIC Statement Programs using the SYSTEM INTRINSIC statement to call MPE/iX intrinsic functions that are incompatible with their MPE V counterparts (MYCOMMAND, CREATEPROCESS, etc.) must modify the calls to those intrinsics. The changed intrinsics generally store procedure labels or addresses in 16-bit variables; these items are 32 bits in MPE/iX. Refer to the MPE/iX Intrinsics Reference Manual and the Application Migration Guide when using these intrinsics. SEGMENT and LOCALITY Directives The LOCALITY directive of FORTRAN 77/iX serves as a synonym for the SEGMENT directive of FORTRAN 77/V. The SEGMENT directive is also retained in FORTRAN 77/iX for compatibility with FORTRAN 77/V. See "LOCALITY Directive" later in this chapter for more information. SYSINTR Directive The MPE V directive SPLINTR has been replaced in MPE/iX by SYSINTR. Overlapping Character Substring Moves FORTRAN 77/V allows overlapping character substring moves to have a ripple effect; FORTRAN 77/iX does not allow this ripple effect. For example, the following code has a different result on FORTRAN 77/V and FORTRAN 77/iX: character ch*10 ch(1:1) = '*' ch(2:10) = ch(1:9) On FORTRAN 77/V, the character string ch is filled with asterisks (*). On FORTRAN 77/iX, the first and second positions contain asterisks and the remainder of the string is undefined. If your FORTRAN 77/V program takes advantage of the ripple effect of overlapping character substring moves, use the STRING_MOVE option with the HP3000_16 directive. (See the section "HP3000_16 Directive" later in this chapter for more information.) If the STRING_MOVE option is used and there are overlapping character substrings, the string is moved one character at a time. If STRING_MOVE is not used, a fast move is done. Therefore, to increase performance, do not use the STRING_MOVE option if your program does not take advantage of the ripple effect of character substring moves.
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