Information is read by giving the MRA or MRB order. Almost 10 ms. is available after a read order is given before information actually enters the TC buffer.
To read a record of unknown length, the read order is first given. The MPS order halts the program until six characters are assembled in the TC information buffer. The next instruction after the MPS, a jump instruction, transfers control from the loop when any flag is set. The next instruction deposits the IO. The record length is determined by not skipping after the MPS order on the setting of the end of record flag. The read-write check flag or the end of record flag is then interrogated to see that the tape is actually at the end of record. If a tape is not at the end of record, then the tape is either at the end of the reel, or a parity check has occurred.
Reading Program
Program to read j binary words into storage beginning in register d, using tape unit 10, j is unknown. The program begins in register enteread.
| enteread | mec | ,clear flags initially |
| mrb 1000 | ,730000001060 | |
| dzm x | ,put zero in memory location x | |
| e | mps | |
| jmp outcheck | ||
| dio x/d | ,store in location modified by x | |
| snx x/+1 | ,add 1 to C(x) | |
| jmp e | ||
| outcheck | mec | ,examine flags |
| spi | ,end of record? | |
| jmp recordend | ,yes | |
| hlt | ,error | |
| recordend | snx x/+1 | ,to find value of j |
| " | ,resume programming C(IA) = j | |
| " | ||
| " | ||
| " |
Forward Spacing
Forward spacing is done by giving an MRB or MRA order. This moves the tape forward with the read-write head positioned at the end of the following record. If n read orders are given, the tape is spaced forward n records. By giving the MEC order, parity flags are examined to see that information on tape has been read correctly.
Backspacing
By giving an MBA or MBB order the tape is moved backwards a record with the read-write heads positioned in the previous end of record gap. The end of record flag is set when the tape has moved backwards a record.