On the page Parsing - Modern Techniques, it was stated that there may be times when TRT might be a better instruction to use than SRST. So what are the pros and cons.

TRT

The Translate and Test Instruction (TRT) is one of the original System/360 instruction, with a single byte within the instruction used to indicate the length of the field to be searched. This limits the length of said field to 256 bytes.

The second argument of a TRT instruction is a 256-byte table containing binary zeroes in all bytes except the locations where the character to be searched for resides in the collating sequence.

For Example:

TRTTAB		DC		256X'0'	DEFINES THE TRANSLATION TABLE.
		ORG		TRTTAB+C' '
BLANK		DC		CL1' '	A BLANK
		ORG		TRTTAB+C','
COMMA		DC		CL1','	A COMMA
		ORG		TRTTAB+C'='
EQUALS		DC		CL1'='	AN EQUALS SIGN
		ORG		,	LOGICAL END OF TABLE

This combination means that more than one character can be searched for at a time. This is useful when Parsing JCL, when a parameter might end with either a comma or a blank.

SRST

The Search String (SRST) instruction is more modern and effectively uses three registers.

The low-order byte of General Purpose Register 0 contains the character to be searched for. Note that this usage is implicit, and is not explicitly defined in the coding of the SRST instruction, which only has two registers on the statement.

The two registers used by SRST are:

• First Operand - Register.
• Can be either even or odd.
• Prior to execution - The address of the character after the last byte to be searched.
• After execution - The address of the found character.
• Second Operand - Register.
• Can be either even or odd.
• Contains the address of the start location for the search.

Because the two registers contain the start and end locations of the search, fields in excess of 256 bytes can be handled. However the use of the low-order byte in Register 0 to hold the character to be found, means that only one character can be searched for at a time.

So which instruction should I use?

Basically it comes down to the size of the field to be searched, and the number of character to be searched for concurrently.

• If the field is in excess of 256 bytes, use SRST.
• If the field is 256 bytes or less in size then either SRST or TRT could be used.
• If multiple characters are to be searched for concurrently, then use TRT.

End of Topic

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