RCA 1802

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The RCA (CDP)1802 (aka RCA COSMAC*, COSMAC 1802) is an 8-bit CMOS microprocessor (µP) introduced by RCA in early 1976, and currently being manufactured by Intersil Corporation. The 1802 has an architecture quite different from most other 8-bit microprocessors.

(* COSMAC is an acronym for COmplementary Silicon Metal-oxide Conductor, which was RCA's term for its first CMOS (COS/MOS) semiconductor technology.)

Contents

[edit] Introduction

The RCA 1802—which is actually the one-chip implementation of the earlier two-chip RCA 1801—has a static CMOS design with no minimum clock frequency, so that it can be run at very low speeds and low power. It has an 8-bit parallel bus with a bidirectional data bus and a multiplexed address bus (i.e., the high order byte of the 16-bit address and the low order byte of the address take turns in using the 8-bit physical address bus lines, by accessing the bus lines in different clock cycles). It has a single bit, programmable output port, and four input pins which are directly tested by branch instructions. Its I/O mode is flexible and programmable, and it has a single-phase clock with an on-chip oscillator. Its register set consists of sixteen 16-bit registers. The program counter (PC) can reside in any of these, providing a simple way to implement multiple PCs, pointers, or registers.

[edit] Applications

From the outset the 1802 has also been available fabricated in Silicon on Sapphire semiconductor process technology, which gives it a degree of resistance to radiation and electrostatic discharge (ESD). Along with its extreme low-power abilities, this makes the chip well-suited in space applications (also, at the time the 1802 was introduced, very few, if any, other radiation-hardened microprocessors were available in the market). The 1802 was used in the Voyager, Viking, and Galileo spacecraft, and has been widely used in Earth-orbiting satellites. The Voyager spacecraft have three 1802s running at 6.4 MHz. All these CPUs sent to space were operating at full military specification temperatures (-55 to +125 °C).

The 1802 was also used in ACAL, a microprocessor based system for the detection of gasses in chemical warfare. ACAL was designed by Oldelft / Delft Instruments, a Dutch company in the military & defence business. ACAL only got as far as prototype stage and never made production.

Commercial applications included the MIL Key building access control system, made in Australia, and marketed by Philips in Europe in the 1980s.

A number of early microcomputers were based on the 1802, including the Comx-35, COSMAC ELF, COSMAC VIP, ELF II, SuperELF, Finnish Telmac_1800 and Yugoslav Pecom 32 and 64, as well as the RCA Studio II video game console (one of the first consoles to use bitmapped graphics).

The first high-level language available for the 1802 was Forth, provided by Forth, Inc. in 1976.

[edit] Technical description

[edit] Registers and I/O

An important feature of the 1802 is the register file of sixteen registers of 16 bits each. Using the SEP instruction, you can select any of the 16 registers to be the program counter; using the SEX instruction, you can select any of the 16-bit registers to be the index register. Register R0 has the special use of holding the memory address for the built-in DMA controller.

The processor has 5 special I/O lines. There's a single Q output that can be set with the SEQ instruction and reset with the REQ instruction. There are four external flag inputs: EF1, EF2, EF3, EF4 and there are 8 dedicated branch instructions to conditionally branch based on the state of those input lines. The EF and Q lines were typically overused on RCA 1802 based hobbyist computers because of the lines' favorable handling. It was typical for the Q line to drive a status LED, a cassette interface, an RS-232 interface, and the speaker. This meant that the user could actually hear RS-232 and cassette data being transmitted.

[edit] Subroutine calls

The processor does not have standard subroutine CALL immediate and RET instructions, though they can be emulated. The register file makes possible some interesting subroutine call and return mechanisms, though they are better suited to small programs than general purpose coding. A few commonly used subroutines can be called quickly by keeping their address in one of the 16 registers; the SEP instruction is used to call a subroutine pointed to by one of the 16 bit registers and another SEP to return to the caller. Before a subroutine returns, it jumps to the location immediately preceding its entry point so that after the SEP instruction returns control to the caller, the register will be pointing to the right value for next time. An interesting variation of this scheme is to have two or more subroutines in a ring so that they are called in round robin order. On early hobbyist computers, tricks like this were commonly used in the horizontal refresh interrupt to reprogram the scan line address to repeat each scan line 4 times for the video controller. Computed subroutine calls were no problem because all CALL instructions were indexed (some processors only had CALL immediate).

[edit] DMA; Clock cycles

The built-in DMA controller is typically used to load or view program memory, depending on the state of the write enable signal, after the processor has been reset but still being held in a special wait state.

Clock cycle efficiency is poor in comparison to most other similar processors. Eight clock cycles makes up one machine cycle. Most instructions take two machine cycles to execute.

[edit] Support chips

[edit] Video

In early microcomputers the companion graphics Video Display Controller chip, CDP1861 for the NTSC video format, (CDP1864 variant for PAL), used the built-in DMA controller to display bitmapped graphics.

The 1861 chip could display 64 pixels horizontally and 128 pixels vertically, though by reloading the R0 register, the resolution could be reduced to 64×64 or 64×32 to use less memory or to make square pixels. Since the frame buffer was similar in size to the memory size, it was not unusual to display your program/data on the screen allowing you to watch the computer "think" (i.e. process its data).

Programs which ran amok and accidentally overwrote themselves could be spectacular. Although the faster version of 1802 could operate at 5 MHz (at 5 V; it was faster at 10 V), it was usually operated at 3.58 MHz/2 to suit the requirements of the 1861 chip which gave a speed of a little over 100,000 instructions per second.

[edit] Code samples

This code snippet tests ALU OPS, it is a diagnostic routine.

..  TEST ALU OPS
0000 90         GHI 0     .. SET UP R6
0001 B6         PHI 6
0002 F829       LDI DOIT  .. FOR INPUT OF OPCODE
0004 A6         PLO 6
0005 E0         SEX 0     .. (X=0 ALREADY)
0006 6400       OUT 4,00  .. ANNOUNCE US READY
0008 E6         SEX 6     .. NOW X=6
0009 3F09       BN4 *     .. WAIT FOR IT
000B 6C         INP 4     .. OK, GET IT
000C 64         OUT 4     .. AND ECHO TO DISPLAY
000D 370D       B4 *      .. WAIT FOR RELEASE
000F F860       LDI #60   .. NOW GET READY FOR
0011 A6         PLO 6     .. FIRST OPERAND
0012 E0         SEX 0     .. SAY SO
0013 6401       OUT 4,01
0015 3F15       BN4 *
0017 E6         SEX 6     .. TAKE IT IN AND ECHO
0018 6C         INP 4     .. (TO 0060)
0019 64         OUT 4     .. (ALSO INCREMENT R6)
001A 371A       B4 *
001C E0         SEX 0     .. DITTO SECOND OPERAND
001D 6402       OUT 4,02
001F E6         SEX 6
0020 3F20 LOOP: BN4 *     .. WAIT FOR IT
0022 6C         INP 4     .. GET IT (NOTE: X=6)
0023 64         OUT 4     .. ECHO IT
0024 3724       B4 *      .. WAIT FOR RELEASE
0026 26         DEC 6     .. BACK UP R6 TO 0060
0027 26         DEC 6
0028 46         LDA 6     .. GET 1ST OPERAND TO D
0029 C4   DOIT: NOP       .. DO OPERATION
002A C4         NOP       .. (SPARE)
002B 26         DEC 6     .. BACK TO 0060
002C 56         STR 6     .. OUTPUT RESULT
002D 64         OUT 4     .. (X=6 STILL)
002E 7A         REQ       .. TURN OFF Q
002F CA0020     LBNZ LOOP .. THEN IF ZERO,
0032 7B         SEQ       .. TURN IT ON AGAIN
0033 3020       BR LOOP   .. REPEAT IN ANY CASE

[edit] External links

Minor parts of this article were originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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