Hardware Programming on the Oric

Fifth edition: a few fixes (keyboard table, microdisc in single-density)
Fourth edition: quick and dirty HTML version
Third edition: updated Second VIA (Telestrat) chapter
Second edition: updated PSG and Disk interface chapters


  1. Introduction: Oric Hardware
  2. Universal Array Logic
  3. Versatile Interface Adapter
  4. Programmable Sound Generator
  5. Keyboard
  6. Printer and joysticks
  7. Screen
  8. Floppy Drive interfaces: Microdisc, Jasmin
  9. Tape
  10. Serial
  11. Real Time Clock
  12. Second VIA (Telestrat)
  13. Appendix: 6502 instruction set for real programmers
  14. Appendix: 6522 VIA data sheet
  15. Appendix: AY-3-8912 PSG data sheet
  16. Appendix: WD 1793 FDC data sheet
  17. Appendix: 6551 ACIA data sheet
  18. Appendix: Intersil ICM 7170 RTC brief reference

Introduction: Oric Hardware

The Oric computers are powered by a 6502 processor running at 1 Mhz. All computers share a central architecture ensuring strong compatibility : this is the Oric-1/Atmos architecture consisting of the cpu, the system bus, a VIA (Versatile Interface Adapter), an UAL (Universal Array Logic) and a PSG (Programmable Sound Generator). I guess many of you know the 65xx processor series, so I won't describe the cpu here. I would just like to point out the processor's NMI line is not connected to anything on the Telestrat (the reset button is really connected to the Reset line) and peripheral request lines may be wire-ANDed on the IRQ line (the versatile interface adapter is the only one connected in the basic configuration) . The UAL is responsible for clocking the cpu and the peripherals as well as managing memory select signals and displaying the screen. The memory select signals allow to disable the internal 16K rom, to enable external IOs or overlay ram, etc. These signals are presented to the system bus only with the basic configuration, so only extensions are able to use them. The VIA is a 6522 one and it is central to the basic peripherals of the Oric: its input-output lines are connected to the keyboard, to the PSG, to the tape, to the printer port (connections are described in next sub-chapter). It also provides 7 sources of interrupts, timers... The PSG is a General Instruments AY-3-8912 (identical to Yamaha's 2910). The CMOS technology of the PSG has obliged the Oric designers to a weird interfacing of the select lines and data lines to the VIA.

Universal Array Logic

The UAL (labelled HSC 10017) saves a lot of components : it builds clock signals, memory signals (so it is a simple memory management unit), and last but not least: video signals. Memory signals are generated when addresses are detected on the bus : the IO is made active when page 3 is addressed and the internal Oric ROM is selected when an address in the C000-FFFF range is accessed. So, all IOs in the basic configuration are done through addresses in page 3. The VIA is always selected with a page 3 address, so the 16 registers of the VIA are viewed 16 times in page 3. For compatibility sake with more complete configurations (drives, joystick, serial interfaces...), it is recommended to access VIA at 0300-030F. Video is programmed by inserting control bytes in-line with screen data. As the UAL refreshes the screen by reading the screen image in memory, internal registers (background and foreground colors, blinking, charsets location...) are updated when those control bytes are encountered (bits 5 and 6 are both 0). See the Screen chapter for a detailed reference.

Versatile Interface Adapter

The 6522 has provision for two eight-bits IO ports (A and B) plus control lines (CA1, CA2, CB1, CB2), interfacing with the other components. Port A is used as a secondary bus : PA0..PA7 lines connect both to the PSG data bus and to the printer port. PSG selection is done thanks to CA2 and CB2 lines. Port B provides many connections to the keyboard, tape, and printer port.

VIA Lines

Oric usage


PSG data bus, printer data lines


printer acknowledge line


PSG BC1 line


keyboard lines-demultiplexer


keyboard sense line


printer strobe line


(not connected)


tape connector motor control


tape connector output


tape connector input



Moreover, the 6522 also features programmable timers, interrupt control and synchronous communication capabilities (synchronous communication is not available in the Oric due to the way the VIA is connected). See the 6522 brief reference in appendix. E.g timer 1 is used by the rom to provide an interrupt every 1/100th a second, decrementing some system variables to allow for the WAIT command, to poll the keyboard or wait a screen refresh. Programs may use both timers, and control interrupts accordingly with the peripheral connections described above (e.g tape reading requires detecting a edge transition on CB1).

Programmable Sound Generator

The AY-3-8912 is a three chanels sound generator allowing simple tones (square waves) and more complex sound using an envelope generator. Moreover, each chanel may be mixed with a noise generator output. On the Oric, the three outputs are wired together and to the loudspeaker. Programming the PSG is rather slow and cumbersome on the oric due to its interfacing to the VIA. As two select lines of the PSG (BC1 and BDIR) are connected to the CA2 and CB2 lines of the VIA, you have to ensure the PCR (Peripheral Control Register) of the 6522 is correctly set in order to communicate with the PSG. As explained before, data to/from the PSG flows through a secondary bus connected to port A of the VIA. Selecting the PSG means asking it to read a register number (selecting one of its register), or reading/writing the previously selected register. Here are the four combinations:






PSG not selected



Read PSG register



Write PSG register



Index register of PSG selected

Last but not least, note that register 14 (0Eh) of the PSG is an IO port (other flavors of the PSG, 8910 and 8913, have 0 or 2 IO ports). On the Oric, this IO port is used to select a column in the keyboard matrix, see the Keyboard chapter.


The keyboard of the Oric computers is just a passive matrix. It is up to the cpu to poll the keyboard by sensing each key in the matrix (so, 64 tests). This is a rather lenghty process due to the way the columns are selected (through the IO port of the PSG). Sensing a key means programming a line (0 to 7) in PB0..PB2 of the VIA, and selecting a column in PSG's IO port (each line of the IO port is connected to a keyboard's column with negative logic, so 11110111 will select column 3), then reading the result in PB3 input (a 1 means the key is pressed). Here is the keyboard matrix (note that the Oric-1 do not have a FCT key):














(right Ctrl)




































Left Shift









FCT (Alt)


















Right Shift





  Please note that the Oric-1 keyboard only has 57 keys, and the Atmos keyboard has 58. This means that eight locations in this matrix were not wired. Euphoric implements additional keys in this matrix in order to better support International keyboards (so, again, those keys in blue in the above table do not exist on the real Oric keyboards).

Printer and joysticks

The Centronics port on the back of the Oric has only 20 pins (10 of them being ground). So there are only 2 control lines : Strobe and Acknowledge. Sending a byte to the printer requires outputting the value through the VIA's port A, then emitting a Strobe pulse (negative edge out to PB4, normal level of PB4 should be 1), and finally waiting for the printer's acknowledge (negative edge received into CA1). As the printer port is bidirectional, it may be used for inputting values too, this is the case when connecting joysticks to a PASE interface on the parallel port. Programming bits 6 and 7 as outputs and bits 0..5 as inputs allows to read the joystick (note that if the PSG has to be written to, then whole port A has to be programmed as output again): bit 7 allows to select left joystick, and bit 6 to select right joystick. Then bits 0, 1, 3, 4 and 5 are used to sense the joysticks switches (respectively left, right, down, up and fire): a 0 means the corresponding switch is activated.


The ULA displays 224 lines of 240 pixels (pixels are not square, they have a 5/4 horizontal/vertical ratio) in 8 colors (1 bit each for red, green, blue). However, specifying background or foreground colors is achieved through in-line control bytes. They are usually called serial attributes because their effect may persist until the end of the line, or even after a complete screen refresh. Serial attributes have both bits 5 and 6 resetted to 0, and allow to set the ULA functionning: hardware blinking, character set selection, double height characters, foreground/background color selection, text/hires mode. The screen image in memory starts either in location A000 or BB80 depending of the current character/bitmap mode. The ULA always starts displaying a line with foreground color being white, background color being black, no blinking, normal height characters and normal character set. It reads 40 bytes in memory to display a 240 pixels line, and serial attributes are processed on the fly. As a byte cannot contains both pixel values and a serial attribute, 6 pixels in the background color will be displayed when encountering a serial attribute (serial attributes take effect immediately, so a background color change will produce 6 pixels of the corresponding color). When in hires mode, the 6 least significant bits in a byte provide 6 successive pixels on screen (a 1 value means foreground color, a 0 means background color). If both bits 6 and 5 in a byte are 0, then the 5 low-order bits are taken as a serial attribute. Bit 7 (most significant bit) is an inversed video flag, meaning that the pixels colors of the byte will be inversed (each reg/green/blue bit is inversed), this does not affect successive bytes. When in text mode, the same 40 bytes are repeatedly read to display 8 lines on screen. The 7 low order bits give an ascii value in the 32..127 range (0..31 values are serial attributes). This value and the current repetition (0 to 7) is used to select a byte in one of the two character set definition tables, whose 6 low- order bits will be interpreted as pixels. Once again, bit 7 is an inversed video flag for the current byte.


X0000XXX: foreground color

X0001XXX: text attributes

X0010XXX: background color

X0011XXX: video mode





Text 60 Hz



Alternate charset


Text 60 Hz



Double height


Text 50 Hz



Double height, alt charset


Text 50 Hz





Hires 60 Hz



Blinking, alt charset


Hires 60 Hz



Blinking, dble height


Hires 50 Hz



Blink, dble height, alt char


Hires 50 Hz

Whatever the current mode is (text or hires), after 200 lines are displayed, the ULA starts reading from memory location BF68 (so there's a 32 bytes hole in hires mode's memory image), and repeats reading 8 times each of the 3 following 40 bytes sequences (so serial attributes switching to text or hires mode, in this area, will only affect the location of character sets (B400-BBFF in text mode, and 9800-9FFF in hires mode) and determine the next starting address of the screen image). Mixing lines of text and lines of hires in the same screen is possible. The ULA adjusts its memory read pointer, and its character sets pointers.

Floppy Drive Interfaces

Drive interfaces for the oric are all driven by the same family of FDC (floppy disc controllers): Western Digital 177x for the Jasmin and new 179x generation for the Microdisc (and Telestrat). Differences in this family exist to connect negative or positive drives, and to allow double-density formats (MFM mode). The FDC 1773 of the Jasmin and the 1793 of the Microdisc are compatible, but the additional circuitry is quite different so the programming routines have to be deeply adapted from one to the other (the telestrat has a second ULA integrating the Microdisc electronics, so it is largely compatible with it). The FDC 1793 (or 1773) is accessible through locations 0310-0313 in Microdisc's electronics, and 03F4-03F7 in Jasmin's electronics. This requires the interface to disable the internal VIA, with the use of the I/O CONTROL line on the system bus. See appendix for FDC programming commands. Microdisc's electronics features an output buffer at location 0314 to select drive number, side number, and memory signals. Also, two signals of the FDC are readable when reading locations 0314 and 0318. The big picture is that the DRQ line (Data Request) has to be polled through bit 7 of location 0318 (a 0 means the FDC is ready to read/write a byte of data), and the end of the command is signaled to the processor as the INTRQ line (interrupt request) of the FDC is connected to the system IRQ line through a NAND gate enabled by bit 0 of location 0314 (a 1 enables interrupts). Alternatively, cpu interrupts may be disabled: in this case, the end of the command can be polled from bit 7 of location 0314 which reflects the INTRQ line, provided that the previous NAND gate is enabled. This is a more bug-proof approach than polling the FDC status register, because reading the FDC status resets the IRQ condition (reading location $0314 doesn't). Microdisc also allows access to the overlay ram (internal ram of the Oric in locations C000-FFFF usually masked by the rom), and to the 8 KB eprom of the interface. This is achieved through bits 7 and 1 of location 0314. This is not implemented in the Telestrat's second ULA as ram overlay has been generalized to 8 memory banks (rom or ram), selected through bits 0..2 of its second VIA.

Write to location 0314 :

bit 7: Eprom select (active low) 

bit 65: drive select (0 to 3) 

bit 4: side select 

bit 3: double density enable (0: double density, 1: single density) 

bit 2: along with bit 3, selects the data separator clock divisor            (1: double density, 0: single-density) 

bit 1: ROMDIS (active low). When 0, internal Basic rom is disabled. 

bit 0: enable FDC INTRQ to appear on read location $0314 and to drive cpu IRQ


Read of location 0314 (only bit 7 connected) :

bit 7: INTRQ state (only if bit 0 above has been set to 1) in negative logic so it's 0 if FDC requests an Interrupt.


Read of location 0318 (only bit 7 connected)

bit 7: DRQ state (active low) 


Jasmin's electronics also features buffers for side/drive selecting, and memory signals, but the DRQ line is connected to the system IRQ line so it allows for interrupt-driven transfers (however, two consecutives bytes are separated by 31.25 micro-seconds, so the interrupt routine has to be fast ! As an example, FT-DOS uses a dedicated interrupt routine, and does not even have time to save registers: the interrupt routine lasts 28 cycles) The end of a command has to be detected by reading the busy bit of the Status Register of the FDC.

location 03F8

bit 0: side select 


location 03F9 : disk controller reset (writing any value will reset the FDC)

location 03FA

bit 0: overlay ram access (1 means overlay ram enabled) 


location 03FB

bit 0: ROMDIS (1 means internal Basic rom disabled) 


locations 03FC, 03FD, 03FE, 03FF : writing to one of these locations will select the corresponding drive


Writing to the tape is simply a question of outputting a square waveform to bit 7 of VIA's B port. So, timing is the essence... The standard system software of the Oric-1/Atmos (no tape software in Telestrat) uses a mix of two frequencies : 2400 Hz and 1200 Hz. For example, in SLOW mode, to write a bit to the tape, the rom routines output either 4 periods at 1200 Hz for a 0 or 8 periods at 2400 Hz for a 1 (so the baud rate is 300). In FAST mode, the difference is much thinner : either a 2400 Hz period for a 1, or half a 2400 Hz period and half a 1200 Hz period for a 0 (so the baud rate lies between 1600 and 2400 depending on the proportion of 1's and 0's). Whatever the mode is (SLOW or FAST), the rom writes one byte as 13 bits on tape: a starting 0, eight bits of data, a parity bit, and three 1 bits. Reading the tape requires measuring the time interval between edges on the CB1 pin (with a VIA's timer of course). In theory, this operation could be interrupt-driven, but I have never seen programs using this feature.


There's no standard serial port on the Oric-1/Atmos but one is standard with the Telestrat and many extensions exist for the Oric-1/Atmos. I have only heard about serial interfaces driven by the 6551 ACIA (Asynchronous Communication Interface Adapter). Four IO addresses allow to program the ACIA, they are located at 031C on the Telestrat. As the Microdisc's electronics does not use these addresses, it would be a good idea if all the serial extensions for Oric-1 or Atmos were located there. See the ACIA brief reference in appendix for a summary of commands (am I tired of writing this or what ?). Note that you need a IRQ handler aware of the ACIA if you enable the ACIA interrupts, otherwise the interrupt condition will not be resetted and the cpu will indefinitely service an interrupt request.

Real Time Clock

Very few Oric owners have this extension. It is most useful for people using their oric as a BBS, since no Oric operating system stores time/date information of files. Moreover, the alarm feature of the main chip is near useless as the extension's designers didn't connect the interrupt request line (so you need to poll the alarm in order to see if time has expired, it is not harder to check if the clock has reached some given time/date). See appendix for reference.

Second VIA (Telestrat)

The Telestrat has a second VIA interfacing new hardware, and allowing the selection of the memory banks. This VIA is located at address 0320. Pin assignment changed from the original Stratos when crossing the chanel (from England to France): as some pins were not connected, the french company decided that a Midi connector could be a nice marketing argument, even with random VIA pins connected behind... Fortunately better additions were done too: capability to use a mouse in right joystick port, phone ring detection (a rather complex circuitry has been added for it...).
So, here is the input/output ports assignment of the final Telestrat board, with its small daughterboards add-ons:

VIA lines

Telestrat usage


Memory bank selection


Minitel port pin 6 / "Midi" port pin 3


RS232/Minitel selection


Third mouse button (right joystick port pin 5)


(Phone ring detection of the Stratos on modem port pin 4 ?) / "Midi" port pin 5


Second mouse button (right joystick port pin 9)


"Midi" port pin 1


not used ?


Joystick ports


Select dipswitch (missing)


Select Left Joystick port


Select Right Joystick port


Phone Ring detection


"Midi" port pin 4


Selecting a memory bank (memory addresses in the C000-FFFF range) is easy, just write the bank number you wish in the three low-order bits of port A. Bank 0 is the overlay ram, banks 1 to 7 may be present due to cartridges inserted in one of the two cartridge slots. Usually, bank 7 contains the TeleMon software, the Telestrat boots on Bank 7 when powered on. Bank 0 is loaded with the StratSed operating system when booting (the system is loaded from the disk in drive). Other banks may contain either ram or rom, from the cartridges inserted.

PB5, PB6 and PB7 allow to select one of three ports to read through PB0..PB4, two of which are joystick ports (PB5 is connected to a missing IC, which was intended to interface to dipswitches (these are absent too)). The switches of the joystick are readable through PB0, PB1, PB2, PB3, PB4 (respectively right, left, fire, down, up). When a switch is closed, a 0 is sensed in the corresponding bit (the pin is connected to ground), otherwise the pin remains not connected in high-impedance input, and a 1 is sensed. Alternatively, a mouse may be connected to the right joystick port : right, left, fire (first mouse button), down, up work as with a joystick (middle and right button are connected to PA7 and PA5).

PA3, PA6, CA1, CB2 are connected to the "Midi" port. I prefer to call it "the fake DIN" since there's no way to use it as a midi connector (IMHO). see my proof in CEOmag 8-) However, how about using this DIN connector to connect a coffee machine or something else ? You've got 4 IO lines, two of them may be used as interrupt lines, this is really nice ! I will soon try to connect a standard PC/AT keyboard there...

PA4 selects which port (RS232 or Minitel) is connected to the ACIA. A 0 selects the Minitel port (TTL levels) and a 1 selects the RS232 port (standard EIA levels, but only the Tx, Rx, -RTS, -CTS, -DCD and -DTR are connected (plus ground pin 7 of course): -DSR is connected to +5V, as the MC1489 can only convert 3 inputs, but I would prefer it grounded. Note also that the original Stratos had -Rx connected to modem port pin 5, and this is converted to Rx with a NAND gate on a Telestrat daughterboard.