The TRT instruction is used to search a block of bytes for one or more values. The format looks like this:

      TRT D1(L,B1),D2(B2)

Here D1(B1) is the address of the variable we want to search, at least L bytes long, and D1(B2) is the address of a search table.

A search table is a table of 256 bytes, initialized as follows:

• A byte whose offset value is a value we hope to find will contain a nonzero value.

• Other bytes are set to 0.

TRT examines the area to be searched, one byte at a time, and looks up the corresponding byte in the table. For instance, if a byte has the value X'48' (decimal 72), TRT looks at the byte in the table at an offset of 72. If that byte is not X'00', the search ends.

Example

Suppose we want to search for a question mark character (X'6F'). The search table we need would look like this:

QMTAB    DC     256X'00'
         ORG    QMTAB+X'6F'
         DC     X'FF'
         ORG

We could use it like this:

         XREAD  BUFFER,80
         TRT    BUFFER(80),QMTAB

where BUFFER is 80 bytes long.

After the TRT: if we found a question mark, register 1 will contain the address of the first question mark character found in BUFFER, the rightmost byte of register 2 will contain the corresponding value from the table (X'FF' in this case), and the condition code will be set to 1 (or 2 if the blank was found at the very end of BUFFER).

If the TRT did not find a question mark character in BUFFER, the condition code will be set to 0.

Notes

The hardest part of using TRT is setting up the search table.

The second hardest part of using TRT is remembering that it can and will change the values in registers 1 and 2.

Suggested Exercises

What would the search table look like if we wanted to find:

• the first upper-case letter?

• the first lower-case letter?

• the first digit (0-9)?

• the first non-blank character?

• the first non-digit character?