Updated: 11 December 1998

VAX MACRO and Instruction Set Reference Manual

Scan Characters

Format

opcode len.rw, addr.ab, tbladdr.ab, mask.rb

Condition Codes

Exceptions

• None.

Opcodes

Description

The assembler successively uses the bytes of the string specified by the length and address operands to index into a 256-byte table whose first entry (entry number 0) address is specified by the table address operand. The logical AND is performed on the byte selected from the table and the mask operand. The operation continues until the result of the AND is nonzero, or until all the bytes of the string have been exhausted. If a nonzero AND result is detected, the condition code Z-bit is cleared; otherwise, the Z-bit is set.

Notes

1. After execution:

   R0 =	Number of bytes remaining in the string (including the byte that produced the nonzero AND result); R0 is zero only if there was no nonzero AND result
   R1 =	Address of the byte that produced the nonzero AND result; if no nonzero result, address of 1 byte beyond the string
   R2 =	0
   R3 =	Address of the table

2. If the string has zero length, condition code Z is set just as though the entire string were scanned.

Skip Character

Format

opcode char.rb, len.rw, addr.ab

Condition Codes

Exceptions

• None.

Opcodes

Description

The character operand is compared with the bytes of the string specified by the length and address operands. Comparison continues until inequality is detected or all bytes of the string have been compared. If inequality is detected, the condition code Z-bit is cleared; otherwise, the Z-bit is set.

Notes

1. After execution:

   R0 =	Number of bytes remaining in the string (including the unequal one) if unequal byte located; otherwise, zero
   R1 =	Address of the byte located if byte located; otherwise, address of 1 byte beyond the string

2. If the string has zero length, condition code Z is set just as though each byte of the entire string were equal to the character.

Span Characters

Format

opcode len.rw, addr.ab, tbladdr.ab, mask.rb

Condition Codes

Exceptions

• None.

Opcodes

Description

The assembler successively uses the bytes of the string specified by the length and address operands to index into a 256-byte table whose first entry (entry number 0) address is specified by the table address operand. The logical AND is performed on the byte selected from the table and the mask operand. The operation continues until the result of the AND is zero, or until all the bytes of the string have been exhausted. If a zero AND result is detected, the condition code Z-bit is cleared; otherwise, the Z-bit is set.

Notes

1. After execution:

   R0 =	Number of bytes remaining in the string (including the byte that produced the zero AND result); R0 is zero only if there was no zero AND result
   R1 =	Address of the byte that produced a zero AND result; if no nonzero result, address of 1 byte beyond the string
   R2 =	0
   R3 =	Address of the table

2. If the string has zero length, the condition code Z-bit is set just as though the entire string were spanned.

9.2.10 Cyclic Redundancy Check Instruction

The operands of the CRC instruction are a string descriptor, a 16-longword table, and an initial CRC. The string descriptor is a standard VAX operand pair of the length of the string in bytes (up to 65,535) and the starting address of the string. The contents of the table are a function of the CRC polynomial to be used. It can be calculated from the polynomial by the algorithm in the notes. Several common CRC polynomials are also included in the notes. The system uses the initial CRC to start the polynomial correctly. Typically, the CRC has the value zero or --1. If the data stream is represented by a sequence of noncontiguous strings, the value would vary from 0 to -1.

The CRC instruction scans the string and includes each byte of the data stream in the CRC being calculated. The instruction includes the byte of the data stream by performing a logical exclusive OR (XOR) with it and the rightmost 8 bits of the CRC. Then the instruction shifts the CRC right 1 bit and inserts a zero on the left. The instruction uses the rightmost bit of the CRC (lost by the shift) to control the logical XOR operation of the CRC polynomial with the resultant CRC. If the bit is a 1, the instruction performs a logical XOR with the polynomial and the CRC. The instruction again shifts the CRC to the right and performs a conditional logical XOR on the polynomial with the result, for a total of eight times. The actual algorithm used can shift by 1, 2, or 4 bits at a time using the appropriate entries in a specially constructed table. The instruction produces a 32-bit CRC. For shorter polynomials, the result must be extracted from the 32-bit field. The data stream must be either a multiple of 8 bits in length or right-adjusted in the string with leading zero bits.

Calculate Cyclic Redundancy Check

Format

opcode tbl.ab, inicrc.rl, strlen.rw, stream.ab

Condition Codes

Exceptions

• None.

Opcodes

Description

The CRC of the data stream described by the string descriptor is calculated. The initial CRC is given by inicrc; it is normally zero or --1, unless the CRC is calculated in several steps. The result is left in R0. If the polynomial is less than order 32, the result must be extracted from the low-order bits of R0. The CRC polynomial is expressed by the contents of the 16-longword table. See the notes for the calculation of the table.

Notes

1. After execution:

   R0 =	Result of CRC
   R1 =	0
   R2 =	0
   R3 =	Address 1 byte beyond the end of the source string

2. If the data stream is not a multiple of 8 bits, it must be right-adjusted with leading zero fill.
3. If the CRC polynomial is less than order 32, the result must be extracted from the low-order bits of R0.
4. Use the following algorithm to calculate the CRC table given a polynomial expressed:

   polyn<n> <- {coefficient of x**{order -1-n}}

   
   The following routine is system library routine LIB$CRC_TABLE (poly.r1, table.ab). The table is the location of the 64-byte (16-longword) table into which the result will be written.

   SUBROUTINE LIB$CRC_TABLE (POLY, TABLE) INTEGER*4 POLY, TABLE(0:15), TMP, X DO 190 INDEX = 0, 15 TMP = INDEX DO 150 I = 1, 4 X = TMP .AND. 1 TMP = ISHFT(TMP,-1) !logical shift right one bit IF (X .EQ. 1) TMP = TMP .XOR. POLY 150 CONTINUE TABLE(INDEX) = TMP 190 CONTINUE RETURN END

5. The following are descriptions of some commonly used CRC polynomials:

   CRC-16 (used in DDCMP and Bisync) polynomial: x^16 + x^15 + x^2 + 1 poly: 120001 (octal) initialize: 0 result: R0<15:0> CCITT (used in ADCCP, HDLC, SDLC) polynomial: x^16 + x^12 + x^5 + 1 poly: 102010 (octal) initialize: -1<15:0> result: one's complement of R0<15:0> AUTODIN-II polynomial: x^32+x^26+x^23+x^22+x^16+x^12 +x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1 poly: EDB88320 (hex) initialize: -1<31:0> result: one's complement of R0<31:0>

6. The CRC instruction produces an UNPREDICTABLE result unless the table is well-formed, like the one produced in note 3. Note that for any well-formed table, entry[0] is always zero and entry[8] is always the polynomial expressed as in note 3. The operation can be implemented using shifts of 1, 2, or 4 bits at a time, as follows:

   Shift (s)	Steps per Byte (limit)	Table Index	Table Index Multiplier (i)	Use Table Entries
   1	8	tmp3<0>	8	[0]=0,[8]
   2	4	tmp3<1:0>	4	[0]=0,[4],[8],[12]
   4	2	tmp3<3:0>	1	all

7. If the stream has zero length, R0 receives the initial CRC.

9.2.11 Decimal String Instructions

The decimal string instructions in this section operate on the following data types:

• Packed decimal string
• Trailing numeric string (overpunched and zoned)
• Leading separate numeric string

Where the phrase "decimal string" is used, it means any of the three data types. Conversion instructions are provided between the data types. Where necessary, a specific data type is identified.

A decimal string is specified by two operands:

1. For all decimal strings, the length is the number of digits in the string. The number of bytes in the string is a function of the length and the type of decimal string referenced (see Chapter 8).
2. The address of the lowest-addressed byte of the string. This byte contains the most significant digit for trailing numeric and packed decimal strings, as well as a sign for leading separate numeric strings. The address is specified by a byte operand of address access type.

Each of the decimal string instructions uses general registers R0 to R3 or R0 to R5 to hold a control block that maintains updated addresses and state during the execution of the instruction. At completion, the registers containing addresses are available to the software for use as string specification operands for a subsequent instruction on the same decimal strings.

During the execution of the instructions, pending interrupt conditions are tested; if any is found, the control block is updated. The first part done is set in the processor status longword (PSL), and the instruction is interrupted (refer to Appendix E). After the interruption, the instruction resumes transparently. The format of the control block at completion is as follows:

The fields ADDRESS 1, ADDRESS 2, and ADDRESS 3 (if required) contain the address of the byte containing the most significant digit of the first, second, and third (if required) string operands, respectively.

The decimal string instructions treat decimal strings as integers with the decimal point assumed immediately beyond the least significant digit of the string. If a string in which a result is to be stored is longer than the result, its most significant digits are filled with zeros.

9.2.11.1 Decimal Overflow

Decimal overflow occurs if the destination string is too short to contain all of the digits (excluding leading zeros) of the result. On overflow, the destination string is replaced by the correctly signed least significant digits of the true result (even if the stored result is --0). Note that neither the high nibble of an even-length packed decimal string nor the sign byte of a leading separate numeric string is used to store result digits.

9.2.11.2 Zero Numbers

A zero result has a positive sign for all operations that complete without decimal overflow, except for CVTPT, which does not change a --0 to a +0. However, when digits are lost because of overflow, a zero result receives the sign (positive or negative) of the correct result.

A decimal string with value --0 is treated as identical to a decimal string with value +0. Thus, for example, +0 compares as equal to --0. When condition codes are affected on a --0 result, they are affected as if the result were +0; that is, N is cleared and Z is set.

9.2.11.3 Reserved Operand Exception

A reserved operand abort occurs if the length of a decimal string operand is outside the range 0 to 31, or if an invalid sign or digit is encountered in CVTSP or CVTTP. The program counter (PC) points to the opcode of the instruction causing the exception.

9.2.11.4 UNPREDICTABLE Results

The result of any operation is UNPREDICTABLE if any source decimal string operand contains invalid data. Except for CVTSP and CVTTP, the decimal string instructions do not verify the validity of source operand data.

If the destination operands overlap any source operands, the result of an operation will be UNPREDICTABLE. The destination strings, registers used by the instruction, and condition codes will be UNPREDICTABLE when a reserved operand abort occurs.

9.2.11.5 Packed Decimal Operations

Packed decimal strings generated by the decimal string instructions always have the preferred sign representation: 12 for "+" and 13 for "--". An even-length packed decimal string is always generated with a "0" digit in the high nibble of the first byte of the string.

A packed decimal string contains an invalid nibble if:

• A digit occurs in the sign position
• A sign occurs in a digit position
• A nonzero nibble occurs in the high-order nibble of the lowest-addressed byte in an even length string

The length of a packed decimal string can be zero. In this case, the value is zero (plus or minus) and 1 byte of storage is occupied. This byte must contain a "0" digit in the high nibble and the sign in the low nibble.

The length of a trailing numeric string can be zero. In this case, no storage is occupied by the string. If a destination operand is a zero-length trailing numeric string, the sign of the operation is lost. Memory access faults do not occur when a zero-length trailing numeric operand is specified because no memory reference occurs. The value of a zero-length trailing numeric string is identically zero.

The length of a leading separate numeric string can be zero. In this case, 1 byte of storage is occupied by the sign. Memory is accessed when a zero-length operand is specified, and a reserved operand abort will occur if an invalid sign is detected. The value of a zero-length leading separate numeric string is zero.

9.2.11.7 Instruction Descriptions

The following instructions are described in this section:

	Description and Opcode	Number of Instructions
1.	Add Packed 4 Operand ADDP4 addlen.rw, addaddr.ab, sumlen.rw, sumaddr.ab, {R0-3.wl}	1
2.	Add Packed 6 Operand ADDP6 add1len.rw, add1addr.ab, add2len.rw, add2addr.ab, sumlen.rw, sumaddr.ab, {R0-5.wl}	1
3.	Arithmetic Shift and Round Packed ASHP cnt.rb, srclen.rw, srcaddr.ab, round.rb, dstlen.rw, dstaddr.ab, {R0-3.wl}	1
4.	Compare Packed 3 Operand CMPP3 len.rw, src1addr.ab, src2addr.ab, {R0-3.wl}	1
5.	Compare Packed 4 Operand CMPP4 src1len.rw, src1addr.ab, src2len.rw, src2addr.ab, {R0-3.wl}	1
6.	Convert Long to Packed CVTLP src.rl, dstlen.rw, dstaddr.ab, {R0-3.wl}	1
7.	Convert Packed to Long CVTPL srclen.rw, srcaddr.ab, {R0-3.wl}, dst.wl	1
8.	Convert Packed to Leading Separate CVTPS srclen.rw, srcaddr.ab, dstlen.rw, dstaddr.ab, {R0-3.wl}	1
9.	Convert Packed to Trailing CVTPT srclen.rw, srcaddr.ab, tbladdr.ab, dstlen.rw, dstaddr.ab, {R0-3.wl}	1
10.	Convert Leading Separate to Packed CVTSP srclen.rw, srcaddr.ab, dstlen.rw, dstaddr.ab, {R0-3.wl}	1
11.	Convert Trailing to Packed CVTTP srclen.rw, srcaddr.ab, tbladdr.ab, dstlen.rw, dstaddr.ab, {R0-3.wl}	1
12.	Divide Packed DIVP divrlen.rw, divraddr.ab, divdlen.rw, divdaddr.ab, quolen.rw, quoaddr.ab, {R0-5.wl, --16(SP):--1(SP).wb}	1
13.	Move Packed MOVP len.rw, srcaddr.ab, dstaddr.ab, {R0-3.wl}	1
14.	Multiply Packed MULP mulrlen.rw, mulraddr.ab, muldlen.rw, muldaddr.ab, prodlen.rw, prodaddr.ab, {R0-5.wl}	1
15.	Subtract Packed 4 Operand SUBP4 sublen.rw, subaddr.ab, diflen.rw, difaddr.ab, {R0-3.wl}	1
16.	Subtract Packed 6 Operand SUBP6 sublen.rw, subaddr.ab, minlen.rw, minaddr.ab, diflen.rw, difaddr.ab, {R0-5.wl}	1

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