Use of stack instead of registers Most current computer instruction sets are based on a general purpose register model. The processor and memory architecture of the classic HP 3000 were based on a stack machine model, like HP's well-known line of RPN calculators. It was said to be inspired by the famous stack-based Burroughs large systems. Rather than having a small number of registers, for example only an AX and BX register in the case of the HP 1000, operands would be pushed on the same stack used to store local variables and return addresses. So rather than LOAD AX, 0X0001 LOAD BX, 0X0002 ADD AX, BX you would have LDI 1 LDI 2 ADD The 16-bit microcoded machines (Series I, II, III, 30, 33, 39, 40, 42, 44, 48, 52, 58, 64, 68, 70, 37, ...) implement a 16-bit word addressed, byte-addressable, segmented, Harvard, Stack Instruction Set Architecture (ISA). Most of the ~214 instructions are 16 bits wide. Stack operations pack 2 per 16-bit word and the remaining few are 32 bits wide. CISC Implementations III: 4 Top of stack registers, 175 ns microinstruction cycle time → 5.7 MHz 30, 33: Silicon on sapphire, 2 Top of stack registers, 90 ns microinstruction cycle time → 11 MHz, instructions take 3-7 cycles 40, 42, 44, 48: Schottky TTL, 4 Top of stack registers, 105 ns microinstruction cycle time → 9.5 MHz 64, 68: ECL, 8 Top of Stack registers, 75 ns microinstruction cycle time → 13 MHz, 8KB cache, 60KB WCS, 2 16-bit ALUs 37: ~8,000-gate CMOS gate array, 4 Top of Stack registers Later 32-bit models used HP's PA-RISC general register-based RISC architecture. PA-RISC Implementations PA-RISC 1.0 Series 925, 930, 935, 949, 950, 955, 960, 980 PA-RISC 1.1 Series 917, 920, 922, 927, 937, 947, 948, 957, 958, 967, 977sx, 987, 990, 991, 992, 995, 918, 928, 968, 978, 988 PA-RISC 2.0 Series 996, A and N class and the 9x9 series HP's exit from the 3000 ecosystem After the enterprise computing market shifted toward commodity Unix systems from a wide range of vendors—systems that HP had also been promoting—in November 2001 Hewlett-Packard announced that a period it called the end-of-life for the HP 3000 would wrap up at the end of 2006, and that no new systems would be sold by HP after 2003. In early 2006, Hewlett-Packard announced that limited vendor support for the HP 3000 would