STRUCTURAL LEVELS OF THE PDP-8 215

signal. Normally, the controller signal causing an interrupt is also connected to the skip input, and skip instructions are used in the software polling that determines the specific interrupting device.

The Data Break input for Direct Memory Access provides a direct access path for a processor or a controller to memory via the processor. The number of access ports to memory can be expanded to eight by using the DM01 Data Multiplexer, a switch. The DM01 port is requested from a processor (e.g., LINC or Model 338 Display Processor) or a controller (e.g., magnetic tape). A processor or controller supplies a memory address, a read or write access request, and then accepts or supplies data for the accessed word. In the configuration (Figure 1), Pc('LINC) and P('338) are connected to the multiplexer and make requests to memory for both their instructions and data in the same way as the PDP-8 processor. The global control of these processor programs is via the processor over the I/O Bus. The processor issues start and stop commands, initializes their state, and examines their final state when a program in the other processor halts or requires assistance.

When a controller is connected to the Data Break or to the DM01 Data Multiplexer, it only accesses memory for data. The most complex function these controllers carry out is the transfer of a complete block of data between the memory and a high speed transducer or a secondary memory (e.g., DECtape or disk). A special mode, the Three Cycle Data Break (described in Chapter 6), allows a controller to request the next word from a block in memory.

The DECtape was derived from M.I.T.'s Lincoln Laboratory LINCtape unit, as indicated in Chapter 7. Data was explicitly addressed by blocks (variable but by convention 128 words). Thus, information in a block could be replaced or rewritten at random. This operation was unlike the early standard IBM format magnetic tape in which data could be appended only to the end of a file.

The ISP of the PDP-8 processor is probably the simplest for a general purpose stored program computer. It operates on 12-bit words, 12- bit integers, and 12-bit Boolean vectors. It has only a few data operators, namely, =, +, minus (negative of), Not, And, Slr l(rotate bits left), Srr 1 (2 rotate bits right), (optional) X, /, and normalize. However, there are microcoded instructions, which allow compound instructions to be formed in a single instruction.

The ISP of the basic PDP-8 is presented in Appendix 1 of this book. The 2¹²-word memory (declared M[0:4095]<0;l1>) is divided into 32 fixed-length pages of 128 words each (not shown in the ISPS description). Address calculation is based on references to the first page, Page.Zero, or to the current page of the Program Counter (PC\Program.Counter). The effective address calculation procedure, called eadd in Appendix 1, provides for both direct and indirect reference to either the current page or the first page. This scheme allows a 7-bit address to specify a local page address.

A 2¹⁵-word memory is available on the PDP-8, but addressing more than 2¹² words is comparatively inefficient. In the extended range, two 3-bit registers, the Program Field and Data Field registers, select which of the eight 2¹²- word blocks are being actively addressed as program and data. These are not given in the ISPS description.

There is an array of eight 12-bit registers, called the Auto.Index registers, which resides in Page.Zero. This array (Auto.Index [0:7] <0 :11>:=M[#l0: #17]<0:l1>) possesses a useful property: whenever an indirect reference is made to it, a 1 is first added to its contents. (That is, there is a side effect to referencing.) Thus, address integers in the register can select the next member of a vector or string for accessing.

The processor state is minimal, consisting of a 12-bit accumulator (AC\Accumulator