The LGP-30 and LGP-21
The LGP-30 is a small computer with an Mp.drum. It is distinct from the first (and succeeding) generation computers using Mp.random_ access and can be described by using the PMS diagram in Fig. 1. The LGP-21, a direct descendant of the LGP-30, having the same ISP, is also described by Fig. 1.
Since there is only one address/instruction, a method is needed for the optimal allocation of operands. Otherwise, each instruction might have to wait a complete drum (or disk) revolution each time a data reference is made. The LGP-30 provides for operand-location optimization by interlacing the logical addresses on the drum so that two adjacent addresses (e.g., 00 and 01) are separated by nine physical locations.1 These spaces allow for operands to be located next to the instructions which use them. There are 64 tracks, each with 64 words (sectors). Each word is accessed by a track address of 6 bits and a word address of 6 bits. The sequence of words (sectors) within a track is 00, 57, 50, 43, 36, 29, 22, 15, 08, 01, 58, 51, 44, 37, . . . , 06, 63, 56, 49, 42, 35, 28, 21, 14, 07, 00. The time between two adjacent physical words is approximately 0.260 millisecond, and the time between two adjacent addresses is 9 x 0.260 or 2.340 milliseconds. The actual maximum t.access is 16.66 ms.2
Half of the instruction (15 bits) is unused. It could be used for extra instructions, indexing, indirect addressing, or a second (+1) address to locate the next instruction, all of which increase the preformance.
Fig. 1. LGP-30 and LGP-21 PMS diagrams.
The ISP, given in Appendix 1 of this chapter, is about the most straightforward in the book. There are only 16 instructions, and the program state is less than two words. Although the performance is limited because of an Mp.cyclic_ access, an Mp.random_ access would serve to make the ISP fairly similar to other faster computers, e.g., an IBM 701.
1The LGP-21 has a space of 18 words.