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doc/pdp10-abi.txt: finish Processor Architecture and Data Representation sections
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Mikael Pettersson
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@ -26,18 +26,18 @@ Machine Interface
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Processor Architecture
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The following documents define the PDP10 architecture:
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[TODO: the 1982 manual]
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[TODO: the KC10 manual]
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[TODO: the XKL-1 manual]
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* DECsystem-10/DECSYSTEM-20 Processor Reference Manual, AD-H391A-T1, Digital Equipment Corporation, 1982.
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* KC10 Functional Description, Digital Equipment Corporation, 1983.
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* TOAD-1 System Architecture Reference Manual, XKL Systems Corporation, 1996.
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Linux for PDP10 requires the following features introduced with the
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KL10B processor:
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* extended addressing
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* G-floating
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* "G format" floating point
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From now on, an unqualified reference in this document to PDP10
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or "the architecture" means a KL10B or KL10B-compatible processor.
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From now on, an unqualified reference to PDP10 or "the architecture"
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in this document means a KL10B or KL10B-compatible processor.
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Programs intended to execute directly on the processor use the PDP10
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instruction set, and the instruction encodings and semantics of the architecture.
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@ -64,7 +64,7 @@ lowest addressed byte position in a storage unit (byte 0).
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Figures 3-1 through 3-3 illustrate the conventions for bit and byte numbering
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within storage units of varius widths. These conventions apply to both integer data
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and floating-point data, where the most significant byte of a floating-point value
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holds the sign and at least the start of the exponent. (TODO: VERIFY FP REP)
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holds the sign and at least the start of the exponent.
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The figures show big-endian byte numbers in the upper left corners, and bit numbers in the
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lower corners.
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@ -101,7 +101,7 @@ Figure 3-3. Bit and Byte Numbering in Doublewords
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Fundamental Types
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Table 3-1 shows how ISO C scalar types correspond to those of the PDP10 processor.
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Figure 3-4 shows how ISO C scalar types correspond to those of the PDP10 processor.
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For all types, a NULL pointer has the value zero.
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Type C sizeof Alignment PDP10
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@ -109,7 +109,7 @@ Type C sizeof Alignment PDP10
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Boolean | _bool 1 1 unsigned byte
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---------------|----------------------------------------------------------------------------------------
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Character | char 1 1 unsigned byte
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| unsigned char 1 1 TODO: check if signed or unsigned should be default
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| unsigned char 1 1
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|----------------------------------------------------------------------------------------
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| signed char 1 1 signed byte
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---------------|----------------------------------------------------------------------------------------
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@ -127,17 +127,23 @@ Integral | int 4 4 signed word
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| unsigned int 4 4 unsigned word
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| unsigned long 4 4
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---------------|----------------------------------------------------------------------------------------
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Long Long | long long 8 4(8?) signed doubleword
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| signed long long 8 4(8?)
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Long Long | long long 8 4 signed doubleword
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| signed long long 8 4
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---------------|----------------------------------------------------------------------------------------
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| unsigned long long 8 4(8?) unsigned doubleword
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| unsigned long long 8 4 unsigned doubleword
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---------------|----------------------------------------------------------------------------------------
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Pointer | any-type * 4 4 unsigned word
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| any-type (*) () 4 4
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---------------|----------------------------------------------------------------------------------------
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Floating-point | TODO
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Floating-point | float 4 4 single precision
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|----------------------------------------------------------------------------------------
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| double 8 4 "G format" double precision
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| long double 8 4 "G format" double precision
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---------------|----------------------------------------------------------------------------------------
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Note - long long is implemented in software, as the architecture's
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double precision fixed point instructions have unsuitable semantics.
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Aggregates and Unions
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Aggregates (structures and arrays) and unions assume the alignment of
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@ -157,14 +163,75 @@ is undefined.
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the structure's alignment. This may require tail padding, depending on
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the last member.
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[TODO: standard 5 examples of struct/union layouts]
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In the following examples, members' byte offsets appear in the upper left corners.
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Figure 3-5: Structure Smaller Than a Word
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Byte aligned, sizeof is 1
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struct { +---------+
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char c; |0 |
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}; | c |
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+---------+
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Figure 3-6: No Padding
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Word aligned, sizeof is 8
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struct { +---------+---------+------------------+
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char c; |0 |1 |2 |
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char d; | c | d | s |
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short s; +---------+---------+------------------+
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long n; |4 |
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}; | n |
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+--------------------------------------+
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Figure 3-7: Internal Padding
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Halfword aligned, sizeof is 4
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struct { +---------+---------+
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char c; |0 |1 |
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short s; | c | pad |
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}; +---------+---------+
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|2 |
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| s |
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+-------------------+
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Figure 3-8: Internal and Tail Padding
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Word aligned, sizeof is 16
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struct { +---------+---------------------------+
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char c; |0 |1 |
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double d; | c | pad |
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short s; +---------+---------------------------+
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}; |4 |
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| d |
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+-------------------------------------+
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|8 |
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| d |
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+------------------+------------------+
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|12 |14 |
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| s | pad |
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+------------------+------------------+
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Figure 3-9: union Allocation
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Word aligned, sizeof is 4
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union { +---------+---------------------------+
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char c; |0 |1 |
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short s; | c | pad |
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int j; +---------+---------+-----------------+
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}; |0 |2 |
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| s | pad |
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+-------------------+-----------------+
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|0 |
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| j |
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+-------------------------------------+
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Bit-fields
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C struct and union definitions may have "bit-fields", defining integral objects
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with a specified number of bits.
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Figure 3-7: Bit-Field Ranges
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Figure 3-10: Bit-Field Ranges
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Bit-field Type Width w Range
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----------------------------------------------------
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@ -190,9 +257,8 @@ unsigned long long | | 0 to 2^W-1
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----------------------------------------------------
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"Plain" bit-fields (that is, those neither signed nor unsigned) always have
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non-negative values. Although they may have
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type char(TODO: depends on char being signed or unsigned by default), short,
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int, long, or long long(?) (which can have negative values), bit-fields of
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non-negative values. Although they may have type short,
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int, long, or long long (which can have negative values), bit-fields of
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these types have the same range as bit-fields of the same size with the
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corresponding unsigned type. Bit-fields obey the same size and alignment
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rules as other structure and union members, with the following additions:
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@ -209,11 +275,88 @@ rules as other structure and union members, with the following additions:
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member, bit-field or other, from residing in the storage unit corresponding
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to the type of the zero-width bit-field.
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The following examples (Figures 3-14 through 3-24) show struct and union
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The following examples (Figures 3-11 through 3-16) show struct and union
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member's byte offsets in the upper left corners. Bit numbers appear in the
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lower corners.
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[TODO: standard 6 examples, adjusted]
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Figure 3-11: Bit Numbering
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+---------+---------+---------+---------+
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|0 |1 |2 |3 |
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0111222333444 | 0111 | 0222 | 0333 | 0444 |
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|0 8|9 17|18 26|27 35|
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+---------+---------+---------+---------+
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Figure 3-12: Left-to-Right Allocation
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Word aligned, sizeof is 4
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struct { +-----+------+--------+-----------------+
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int j:5; |0 | | | |
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int k:6; | j | k | m | pad |
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int m:8; |0 |5 |11 |19 |
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}; +-----+------+--------+-----------------+
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Figure 3-13: Boundary Alignment
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Word aligned, sizeof is 12
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struct { +----------+----------+-------+---------+
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short s:10; |0 | | | |
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int j:10; | s | j | pad | c |
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char c; |0 |10 |20 |27 |
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short t:10; +----------+--------+-+-------++--------+
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short u:10; |4 | | | |
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char d; | t | pad | u | pad |
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}; |0 |10 |18 |28 |
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+---------++--------+----------+--------+
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|8 | |
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| d | pad |
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|0 |9 |
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+---------+-----------------------------+
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Figure 3-14: Storage Unit Sharing
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Halfword aligned, sizeof is 2
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struct { +---------+---------+
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char c; |0 |1 |
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short s:9; | c | s |
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}; |0 |9 |
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+---------+---------+
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Figure 3-15: union Allocation
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Halfword aligned, sizeof is 2
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union { +---------+---------+
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char c; |0 |1 |
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short s:9; | c | pad |
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}; |0 |9 |
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+---------+---------+
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|0 |1 |
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| s | pad |
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|0 |9 |
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+---------+---------+
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Figure 3-16: Unnamed Bit-Fields
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Byte aligned, sizeof is 9
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struct { +---------+---------------------------+
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char c; |0 |1 |
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int :0; | c | :0 |
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char d; |0 |9 |
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short :10; +---------+---------+----------+------+
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char e; |4 |5 |6 | |
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char :0; | d | pad | :10 | pad |
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}; |0 |9 |18 |28 |
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+---------+---------+----------+------+
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|8 |
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| e |
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|0 |
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+---------+
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Note - In Figure 3-16, the presence of the unnamed int and short fields do not
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affect the alignment of the structure. They align the named members relative
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to the beginning of the structure, but the named members may not be aligned in
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memory on suitable boundaries. For example, the d members in an array of this
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structure will not all be on an int (4-byte) boundary.
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As the examples show, int bit-fields (including signed and unsigned) pack more
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densely than smaller base types. You can use char and short bit-fields to force
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