The MAC system programs, MACDMP, TECO, and MIDAS, assume a certain data structure on DECtapes which they handle. Each DECtape has 1100 blocks of 200 words, numbered 0 through 1077. Block 0 and blocks 1070 through 1077 are not used by the MAC system. Block 100 of each tape contains the File Directory: a 200-word table describing the current contents of blocks 1 through 1067. The data on the tape is organized into files, each file consisting of one or more blocks. Each file has a name and a mode: the name is composed of 2 six-character subnames, and the mode is a two-bit number. The File Directory has space for 27 files.
