312 THE PDP-11 FAMILY

The second power-up option causes an unconditional entry to the ASCII console routines. This allows remote system startup without the necessity of controlling the bus Halt line. The processor may then be started, as usual, by an ASCII console command.

The last two options allow program execution to begin at a specified address in either macrocode or microcode. Option three sets the macro PC to 173 000 octal and starts normal execution. Option four causes a jump to microcode location 3002 octal, in the fourth MICROM page. Here, the CPU expects to find a user-written microcode routine to perform a special power-up sequence. The state of the BHALT line is not checked in this last case until the execution of the first macrocode instruction is completed.

The Maintenance Instruction. For ease in hardware checkout, a special maintenance instruction is included in the LSI-l 1 repertoire. This instruction stores the contents of five internal registers in a specified block in the main memory. The information may then be used by a diagnostic program to probe the internal operation of the microlevel processor.

Upward Compatibility. Because the basic instruction set of the LSI- 11 processor is that of the entire PDP-l1 family, the user has an extremely large range of compatible processing systems at his disposal. This range extends from the LSI-l 1 on the low end to the PDP-l 1/70 on the high end. The consistency of the instruction set provides economies in training and documentation costs, as well as the ability to carry specific application programs, or even complete operating systems, from one family member to another. Thus, a user currently employing a small PDP-11, like the PDP-11/05, can easily convert to the low cost LSI- 11 without losing a past investment in software development. This compatibility also eases the program development problems often associated with microcomputer systems; assembly, compilation, and initial debugging may be done on any PDP-l 1 system, with the generated code loaded into an LSI-l1 system for testing and final debug. Through the use of the LSI-l1 ASCII console, a central PDP-l1 system may initialize, load, and start up a remote LSI-l1 system over an asynchronous serial line or other link.

Software Support. Other members of the PDP-l 1 family, beginning with the Model 20 (Chapter 9), have been in service for some time. Thus the system designer has at immediate hand a large number of language processors, utility routines, and application programs. Many of these programs will run with little or no modification on an LSI-l1 system. This existing library of software provides the user with a head start in the application of micro computers, at little or no development cost.

Network Capability. Since the LSI-11 shares a common set of data-types and file structures with other PDP-l1 systems, many communication problems disappear. When linked through line protocols such as DDCMP (digital data communications message protocol [DEC, 1974; DEC, 1974a]), LSI-11s may ex change programs and files with other PDP-11s without adjustments for differing word sizes, operating systems, file structures, etc. This fact makes the LSI-l1 the ideal choice for a network node processor. Used with distributed programming systems such as RSX-l 1, RSTS, or RT-l1, the individual LSI-l1 processors may not even require their own mass storage devices, but rather share those of other network nodes. A monitoring network might then consist of a large central PDP-l 1 with disks, magnetic tape units, and other peripherals, together with several remote LSI-11s which would directly control transducers and communication lines. Yet, even in such a functionally differentiated sys em, all processors would be homogeneous in