An extra improvement to the Ramdisk is RAMBO mode. It requires an extra piece of hardware but allows the user to access the SRAM direcly, as 8K pages that appear in the area >6000-7FFF. The DSRs make sure a RAMBO page will never overlap with virtual disk data.
The original Horizon Ramdisk was designed by Ron Gries. Two other versions, the Horizon 2000 and the Horizon 3000, were subsequently released by Bud Mills at:
Western Horizon Technologies 11445 Clayton Road San Jose, CA 95127 (408)-259-4411 http://www.whtech.com/
Hardware
CRU logic
Memory selection
Page selection
The 1-91 revision
Power supply
Phoenix mod
Memory expansion
RAMBO board
Software
Ramdisk Operating System
DSRs
Subprograms
Power-up
ISR
Personnal additions
Other programs
Test programs
Configuration
Demo
The board could be bought either preassembled and tested, or as a kit. It came with a very detailed manual on how to build and test it. Click here for a commented picture and here for the back of the card (not very interesting, but someone requested it, so here it is)..
The CRU address is processed by a 74LS138 decoder that decodes memory lines A5-A7. Line A4 is connected to the enabling pin G2A* (and must therefore be low), whereas A3 is connected to G1 (and must be high). CRU operations are identified by a low signal on the CRUCLK* line, connected to the G2B* selection pin. Note that CRUCLK* is only active during CRU output operations, but since the Ramdisk does not allow CRU input this design is ok.
The 8 switches of the DIP switch are connected together on one side and to the decoder outputs Y0* through Y7* on the other. To select a CRU address, one of the switches must be closed, all the other left open.
The output of the DIP switch is further combined with A11, via another 74LS138, wich allows for the use of 16 CRU bits, instead of the usual 8. The outputsY0* and Y1* of this second decoder each enable a 74LS259 addressable latch. From here, we have a typical CRU circuit: the 74LS259s decode address lines A12-A14, take their data input from A15/CRUOUT and are reset by the RESET* line.
CRU bit 0 is used by the memory selection circuit, and also powers a
2N2222 transistor that lights up a red LED visible from the front panel.
CRU bit 15 is used by the RAMBO board, all other bits serve as page selection
lines for the SRAM.
74LS138 #1 74LS138 #2 74LS259 #1 ,-|<|--+5V +--------+ +--------+ +--------+ |/ LED A7>---|S0 Y0*|--o-o--, A11>---|S0 Y0*|------, A12>---|S2 Q0|----| A6>---|S1 Y1*|--o o--+ Gnd----|S1 Y1*|---, | A13>---|S1 Q1|- |\ A5>---|S2 Y2*|--o o--+ Gnd----|S2 | | | A14>---|S0 Q2|- '-www-- Gnd | Y3*|--o o--+ | | | | | Q3|- 100 Ohms CRUCLK*>---|G2B* Y4*|--o o--+----------|G2B* | | | A15>---|D Q4|- A4>---|G2A* Y5*|--o o--+ Gnd----|G2A* | | | | Q5|- A3>---|G1 Y6*|--o o--+ +5V----|G1 | | '---------|EN Q6|- | Y7*|--o o--' | | | RESET*---|RST Q7|- cru bit 7 +--------+ +--------+ | +--------+ | 74LS259 #2 | +--------+ | A12>---|S2 Q0|- cru bit 8 | A13>---|S1 Q1|- | A14>---|S0 Q2|- | | Q3|- | A15>---|D Q4|- | | Q5|- '------------|EN Q6|- RESET*---|RST Q7|- cru bit 15 +--------+ |
A 74LS138 decodes lines A0 through A2 of the address bus in chunks of >2000 bytes. The G2A* input of the decoder is enabled by the MEMEN* line, so as to exclude CRU operations. The G1 input is pulled up at +5V by a resistor, but can be grounded by with an external switch: this prevents the card from answering to any memory reques (very handy when something messed up the DSRs).
The Y2* output of the decoder is activated by memory accesses in the range >4000-5FFF. It is used to enable another decoder, a 74LS156 that decodes address lines A3 and A4, as well as CRU bit 0. Three of the decoder outputs, Y0*, Y1*, and Y2* are connected together and pulled up to +5V by a 2.7K resistor. This "wired or" design results in a selection line that reacts to memory operations in the range >4000-57FF, without requiring extra OR gates. This line is connected to the CS1* chip selection pin of the 6264 memory chip.
Output Y3* is similarly used to select the SRAMs for any access in the range >5800-5FFF. Note the weird way these two selection signals are combined with diodes to generate the selection signal RDBENA* that enables the data bus drivers in the connection card and cable. A similar circuit, with an additional 2.7K pull-up to +5 volts, enables the 74LS245 that buffers the data bus on the Ramdisk. By the way, the address lines A3-A14 and the WE* line are buffered by two 74LS144.
Pin CS2 of the 6264 is connected to +5V via a LED, which is not visible
on the front panel of the PE-box, but can be checked by opening the box.
It's also grounded by a 2.7K resistor. When the power is off, a low level
on this pin will put the 6264 in a high impedance state, which saves power
(the 6264 is powered by a battery-backed supply).
74LS138 #4 74LS156 6264L15 +--------+ +--------+ 2.7K +--------+ A2>---|S0 Y0*| CRU bit 0--|S2 Y0*|---+--www--+5V A15>---|A0 D0|---<D5 A1>---|S1 Y1*| A5>---|S1 Y1*|---+ A14>---|A1 D1|---<D6 A0>---|S2 Y2*|---, A5>---|S0 Y2*|---+----------, A13>---|A2 D2|---<D7 | Y3*|-, | | Y3*|-------, | A12>---|A3 D3|---<D0 MEMEN*>---|G2B* Y4*| | '-----------|G1 | | | A11>---|A4 D4|---<D1 Gnd---|G2A* Y5*| | | | | | A10>---|A5 D5|---<D2 +5V--www-+-|G1 Y6*| '--Rambo sel | | | | A9>---|A6 D6|---<D3 2.7K | | Y7*| | | | | A8>---|A7 D7|---<D4 Gnd--o o-' +--------+ +--------+ | 1N34A| A7>---|A8 | ,------|--|>|-+ A6>---|A9 W*|---<WE* RDBEN*>----------------+--|>|-+ | A5>---|A10 | 2.7K | | A4>---|A11 G*|---Gnd +5V---www---+------|--|>|-+ A3>---|A12 | 74LS245 G*-----------------+--|>|-+ | | | 1N34A | '---------|CS1* | | +5V--|>|---+---|CS2 | SRAMsel---------' LED | +--------+ Gnd--www---+ 2.7K 2.7K : PN2222 ,-www----+5V :(1) | ,--+---Voff '---www--|/ (1) Rev 1/91 only 2.2K |\___Gnd |
74LS138 #3 2x 74HC154 upto 32 x SRAM +--------+ +--------+ +--------+ X0----|S0 Y0*|------, X2----|S0 |-+ A15>---|A0 |-+ X1----|S1 Y1*|----, | X3----|S1 Y0*| | A14>---|A1 D0| |---<D5 Gnd---|S2 Y2*|pig | | X4----|S2 Y1*| | A13>---|A2 D1| |---<D6 | Y3*|pig | | X5----|S3 Y2*| | A12>---|A3 D2| |---<D7 Gnd---|G2B* Y4*| | | SRAMsel---|G1* Y3*| | A11>---|A4 D3| |---<D0 (2) Gnd---|G2A* Y5*| | | (1) | Y4*| | A10>---|A5 D4| |---<D1 +5V---|G1 Y6*| | | | Y5*| | A9>---|A6 D5| |---<D2 | Y7*| | | | Y6*| | A8>---|A7 D6| |---<D3 +--------+ | | | Y7*| | A7>---|A8 D7| |---<D4 | | | Y8*| | A6>---|A9 W*| |---<WE* | | | Y9*| | A5>---|A10 | | | | | Y10*| | CRU bit 2---|A11 OE*| |---Gnd | | | Y11*| | CRU bit 1---|A12 | | | | | Y12*| | CRU bit 4---|A13 | | | | | Y13*| |etc CRU bit 3---|A14 | | | | | Y14*|------, X6-------|A15 | | | '-----------|G2* Y15*|----, | X7-------|A16 | | | +--------+ | | | X8-------|A17 | | '--------------|G2* | | | X9-------|A18/CS2 | | +---------+ | | +5Vbat----|CS2/A18 | | | '------------|CS1* | | (1) Voff on 1/91 revision | etc +--------+ | (2) SRAMsel on 1/91 revision +---------+ |
Several types of SRAM chips can be used, depending how much memory you need... and how much money you've got. The common point is that they should be capable of low-power stand-by so that they can be maintained by batteries while the PE-box power is off.
The following chips are recommended by WHT:
Due to space constraints, only 12 SRAM chips can be fitted on the
board. It is however possible to "piggy-back" two additional layers of
12 chips, i.e. to solder them directly on top of the others, with only
their CS1* pin connected to a different trace on the PCB. Alternatively,
WHT sold a piggy-back board that supports the extra chips without having
to stack them up Presumably, this board contains two additional 74HC154
decoders (see connections labelled "pig" in the schematic) and sockets
for more memory chips, but I have never seen it.
All this adds up to 1.125 megabytes with 32 Kbytes chips (3 x 12 x 32K), 4.5 Megs with 128 Kbytes chips (3 x 12 x 128K) and 18 Megs with 512 Kbytes chips.
The CRU bits issued by the two 72LS259 must be connected differently,
depending on how many chips you have, and what their size is. Note that
it is not possible to mix chips of different sizes. The Horizon 3000 ramdisk
board comes with jumpers that can be installed so as to wire the CRU bits
correctly. The connections are summarized in the table below ( X0-X9 refer
to the corresponding lines in the above schematic).
| CRU bit | 32K chips | 128K chips | 512K chips |
|---|---|---|---|
| 1 | SRAM: A12 | SRAM: A12 | SRAM: A12 |
| 2 | SRAM: A11 | SRAM: A11 | SRAM: A11 |
| 3 | SRAM: A14 | SRAM: A13 | SRAM: A13 |
| 4 | SRAM: A13 | SRAM: A14 | SRAM: A14 |
| 5 | 74HC154: S0 (X2) | SRAM: A16 (X7) | SRAM: A16 (X7) |
| 6 | 74HC154: S1 (X3) | SRAM: A15 (X6) | SRAM: A15 (X6) |
| 7 | 74HC154: S2 (X4) | 74HC154: S0 (X2) | SRAM: A17 (X8) |
| 8 | 74HC154: S3 (X5) | 74HC154: S1 (X3) | 74HC154: S0 (X2) |
| 9 | 74LS138: S1 (X1) | 74HC154: S2 (X4) | 74HC154: S1 (X3) |
| 10 | 74LS138: S0 (X0) | 74HC154: S3 (X5) | 74HC154: S2 (X4) |
| 11 | not used | 74LS138: S0 (X0) | 74HC154: S3 (X5) |
| 12 | not used | not used | 74LS138: S0 (X0) |
| 13 | not used | not used | SRAM: A18 (X9) |
CRU bit 0 is used to select the Ramdisk card and make it appear in the
>4000-5FFF address range.
CRU bit 14 is not used.
CRU bit 15 is used by the RAMBO board, that makes the SRAMs appear
in the >6000-7FFF address range.
Connection X1 is grounded when not used.
In the above schematics, it is shown that the 1-91 revision now controls the SRAMs at the level of the master 74LS138, rather than through the 74HC154. The result is the same, but it frees the G1* enabling input of the 74HC154, so that it can be use to disable the SRAMs when power is off.
This Voff signal is described in the previous schematics. It is generated by a PN2222 transistor, placed between a 2.7K pull-up resistor to the battery-backed power and the ground. The base of this transistor is connected to the regular +5 volts, via a 2.2K resistor and the diagnostic LED. When power is off, the transistor is passing and the 74HC154s are enabled (G1* is connected to ground). When power is off, the transistor is blocking and the battery-backed power disables the 74HS154s through the pull-up resistor.
A critical feature of the Ramdisk is that it contains batteries that preserves its content while the power is off. These can either be three AAA nickel-cadmium rechargable batteries, or three AAA dry batteries, or a 4.5 volts lithium battery. The nice thing with NiCad batteries is that they are automatically recharged when the power is on. So, unless you leave your PE-box off for several weeks, you never have to worry about changing batteries. However, NiCad are meant to be fully discharged, then fully recharged, which is not what's going to happen within the Ramdisk. This may eventually lead to battery failure... Dry batteries are easier to find but will need to be replaced after about a year. A lithium cell should last for at least five years.
To save power, only part of the Ramdisk is connected to the battery-backed power line: the power lines for the SRAMs, the 6264 RAM and the two 74HC154 decoders (that's why these are HC, not LS: we need CMOS chips, not TTL). Note the diodes, that prevent battery current from going to the 7805 to the batteries and conversely. The ideal diodes for this purpose should have very low leakage current: FDH300 (Fairchild) or 1N3595 (equivalent). This being said, I have installed a 1N4148 with dry batteries and it seems to work fine.
Of course, when using NiCad batteries, diode (1) must be removed so that the batteries can charge when the power is on. Instead, you can install a second current-limiting resistor of 33 Ohms.
N.B. The diode on the output of a 7805 regulator will cause a small
drop in voltage (about 0.6V). To compensate for this, an identical diode
is present at the REF input of the regulator, which bumps up the voltage
accordingly.
7805 +--------+ 1N4004 +8V---+---+----+----|In Out|---+----|>|----+-------------> +5V | | | | Ref | | '---||---Gnd | | | +---+----+ | 1N914 10 uF 100nF = = 10 | | '----|>|----+-------------> +5V bat | | uF '--www---+---|>|---Gnd | Gnd Gnd 2.7K 1N914 +---||---Gnd | 10 uF Batteries FDH300 | Gnd-----www-------|'|'|'---------|>|----' Diode with Li or dry bats 33 Ohms or ---www---- Resistor with NiCad bats 33 Ohms |
The Phoenix modification splits the Ramdisk memory in two chunks, and dedicates one to the bootdrive and one to the ramdrive. It's like having two Ramdisks on the same board, a big one and a small one. The total can only add up to 1056K though, not 3 Megs as is the case for the normal Horizon Ramdisk. Each "drive" can have its own CRU address, but the Phoenix design also allows to have them both on the same CRU address. This is done by counting the number of CRU bits passed to the Ramdisk: 8 bits (or less) will access the bootdrive, 16 bits will access the ramdrive.
With a Ramdisk that uses 128 Kbytes SRAM chips, the modification consists in taking two of the 128K SRAM chips out of the control of the 74HC154 decoder #1 and transfering them to the second 74HC154 decoder (which is not used for Ramdisk less than 1 Meg in size). These two chips will make up the 256K bootdrive. The remaining chips, controlled by decoder #1, make up the ramdrive. A slight modifications the CRU bits 5 to 7 is required, so as to maintain a continuous address space. This is shown on the schematic below.
When using 32 Kbytes SRAM chips, the two existing 74HC154 decoders are left untouched but a third 74HC154 must be piggybacked on #1, and wired as #2 in the schematic below. This is because #2 is already in use with 32K chips. The memory for the bootdrive will come from 3 to 8 extra 32 Kbytes chips controlled by decoder #3. These must be piggybacked on the existing SRAM chips, which are already piggybacked: that means stacking them 3 high. This modification is outlined in the schematic below by comments in [square brackets].
The Horizon 3000 board already includes a socket for an extra 74LS00 chip. This chip contains 4 NAND gates, two of which are wired as a AND gate, so as to combine two different CRU addresses. One for the ramdrive which is selected with the DIP switch, and one for the boot drive which is hardwired as either >1400 or >1600.
The other two NAND gates on the 74LS00 are wired as a flip-flop that reacts to CRU operations on bits 0-7 and 8-15 respectively. One output of the flip-flop enables the 74LS138 decoder that selects the 74HC154 decoder #1, the second output controls the 74HC154 decoder #2 directly (or the extra decoder, #3, in case of a 32K-chips Ramdisk).
Finally, the 6264 SRAM that contains the ROS has to be replaced with
a 32 Kbyte chip (only 16K are used, but I don't think Hitachi makes 16K
chips anyhow). The additional address line (i.e. A14) is controlled by
the flip-flop, so that the DSR space can hold a different copy of the ROS
for each partition.
,----------+---------------------------, 74LS138 #1 | | 74LS138 #2 | +--------+ | | +--------+ | A7>---|S0 Y0*|-|-o-o--, | A11>---|S0 Y0*|-----|---+----->74LS259 #1 (bits 0-7) A6>---|S1 Y1*|-|-o o--+ | Gnd----|S1 Y1*|-, |_? | A5>---|S2 Y2*|-|-o o--+ | Gnd----|S2 | | ,--=|)o--+------------------, | Y3*|-|-o o--+ | | | | ,--|--------' | CRUCLK*>---|G2B* Y4*|-+-o o--+ | 74LS00 | | | | '----, ,---->A14/32K | A4>---|G2A* Y5*|---o o--+---=|)o--=|)o---|G2B* | | ,=|)o---+------+ SRAM | A3>---|G1 Y6*|---o o--+ Gnd----|G2A* | +-' '----, (new) | | Y7*|---o o--+ +5V----|G1 | | ? | | +--------+ | +--------+ '--|--->74LS259 #2 | | '------------------------------' (bit 8-15) | | | | ,-----------------------------------------------------------------------------' | | 74LS138 #3 74HC154 #1 [and #2] 74HC154 #2 [or #3] | | +--------+ +--------+ +--------+ | | X0----|S0 Y0*|------, X2----|S0 | 128K SRAMs X2----|S0 | | | X1----|S1 Y1*|--, | X3----|S1 Y0*|--->CS1* X3----|S1 Y0*|--->CS1* | | Gnd---|S2 Y2*| | | X4----|S2 Y1*|--->CS1* X4----|S2 Y1*|--->CS1* | | | Y3*| | | X5----|S3 Y2*|--->CS1* X5----|S3 Y2*|nc SRAMs | '--------|G2B* Y4*| | | SRAMsel---|G1* Y3*|--->CS1* SRAMsel---|G1* Y3*|nc 2x 128K | Gnd---|G2A* Y5*| | | | Y4*|--->CS1* | Y4*|nc[or 8x 32K]| +5V---|G1 Y6*| | | | Y5*|--->CS1* | Y5*|nc | | Y7*| | | | Y6*|--->CS1* | Y6*|nc | +--------+ | | | Y7*|--->CS1* | Y7*|nc | | | | Y8*|--->CS1* | Y8*|nc | | | | Y9*|--->CS1* | Y9*|nc | nc | | Y10*|nc (now Y0*/#2) | Y10*|nc | [or 74HC154 #2] | | Y11*|nc (now Y1*/#2) | Y11*|nc | | | Y12*|nc | Y12*|nc | | | Y13*|nc [or 32x 32K] | Y13*|nc | | | Y14*|nc | Y14*|nc | '-----------|G2* Y15*|nc ,---|G2* Y15*|nc | +--------+ | +--------+ | '--------------------------' |
This way of counting CRU bit may seem a bit overcomplicated. After all, why not have a different CRU address for each "drive"? This can actually be done, thanks to two SPDT switches that are place on the inputs of the flip-flop (marked as ? in the above schematic). The switches allow to use a Phoenix-modified Ramdisk in a TI-99/4A system, that always uses 16 bits to access a Ramdisk, by having a different CRU address for each partition. In this case there is no need to limit the memory to 800K + 256K: each drive could comprise up to 16 128K chips, but the extra memory won't be accessible from a Geneve.
When in "Geneve" position, the switches connect the flip-flop to Y0*
and Y1* of the 74LS138 #2, as shown above. When in the "TI-99/4A" position,
the switches connect the flip-flop to the CRU address selection lines:
the output of the DIP-switch and pin Y4* (or Y6*) of the 74LS138 #1. Note
that in this case the address on the DIP switch must be different from
the hardwired one.
| CRU bit | 32K chips | 128K chips |
|---|---|---|
| 1 | SRAM: A12 | SRAM: A12 |
| 2 | SRAM: A11 | SRAM: A11 |
| 3 | SRAM: A14 | SRAM: A14 |
| 4 | SRAM: A13 | SRAM: A13 |
| 5 | 74HC154: S0 (X2) | 74HC154: S0 (X2) |
| 6 | 74HC154: S1 (X3) | SRAM: A16 |
| 7 | 74HC154: S2 (X4) | SRAM: A15 |
| 8 | 74HC154: S3 (X5) | 74HC154: S1 (X3) |
| 9 | 74LS138: S1 (X1) | 74HC154: S2 (X4) |
| 10 | 74LS138: S1 (X1) | 74HC154: S3 (X5) |
| 11 | not used | 74LS138: S0 (X0) |
| 12 | not used | 74LS138: S1 (X1) |
| 13 | not used | not used |
As far as software is concerned, the Phoenix modification requires a small program by J. Schoeder, RAMDOS, that patches the Geneve's MDOS so that it can see the Horizon Ramdisk. Each time RAMDOS is run, it toggles between "boot disk" and "ramdisk", but does not allow for simultaneous use of the two partitions.
The 74LS08 must be piggy-backed on top of the CRU address decoder 74LS138, but that's just for power-supply: all pins but ground and Vcc are bent out. By contrast, the SRAM comes on top of the 6264 SRAM chip that contains the DSRs, with all pins (but Vcc, CS1*, A13 and A14) connected to those of the 6264.
The two diodes are added to the RDBENA* selection circuit, and to the
74LS145 data bus buffer selection circuit respectively. All other connections
are made with thin wires to the bent out pins of the 74LS08.
74LS138 #4 (existing) 62256 (new) +--------+ +--------+ A2>---|S0 Y0*| A15>---|A0 D0|---<D5 A1>---|S1 Y1*|---------------------, A14>---|A1 D1|---<D6 A0>---|S2 Y2*|---Ramdisk sel | A13>---|A2 D2|---<D7 | Y3*|---Rambo sel | A12>---|A3 D3|---<D0 MEMEN*>---|G2B* Y4*| | A11>---|A4 D4|---<D1 Gnd---|G2A* Y5*|------, | A10>---|A5 D5|---<D2 +5V--www-+-|G1 Y7*|---+--==|)--+---, | 74LS08 (new) A9>---|A6 D6|---<D3 | | Y6*|---'=|)---+-|---=|)--=|)---------------, A8>---|A7 D7|---<D4 Gnd--o o-' +--------+ | '--------------------------|----------|A13 | '----------------------------|----------|A14 | | A7>---|A8 | 2 existing diodes----+------|>|----+ A6>---|A9 W*|---<WE* RDBENA*>-----------------------' 1N34A (new)| A5>---|A10 | | A4>---|A11 G*|---Gnd 2 existing diodes-----+------|>|----+ A3>---|A12 | 74LS245 G*------------------------+ 1N34A (new)| | Vcc|---+5V +5V---www---' '----------|CS1* | +--------+ |
The existing 74LS138 decoder divides the memory space in chunks of >2000 bytes. Its output Y1* is active (low) during memory operations in the range >2000-3FFF (low memory expansion). Similarly, Y5*, Y6* and Y7* reacts to >A000-BFFF, >C000-DFFF and >E000-FFFF respectively.
The new 74LS08 chip contains four AND gates. They are used to combine Y1*, Y5*, Y6* and Y7* so that a memory operation in the range reserved for the memory expansion card will activate the CS1* selection pin of the newly installed 62256 SRAM. The same selection signal is used to activate the RDBENA* line (that enables the data bus buffers in the connection cable) and the existing 74LS245 (the data bus buffer on the Ramdisk card). Two diodes are used to combine this signal with the existing ones, in a "wired and" fashion.
All data and address lines of the 62256 are connected to the corresponding pins on the 6264 on which it is piggy-backed. There are however two extra address lines to take care of. One is connected to the Y6* AND Y7* signal line, the other to the Y5* AND Y7* signal line, effectively decoding the 32K address space of the memory expansion:
>2000: A13=H A14=H >A000: A13=L A14=H >C000: A13=H A14=L >E000: A13=L A14=LFinally, note that the newly installed SRAM takes its power from the regular +5V, not the battery-backed one used by the 6264. This is because the memory expansion is not supposed to retain its content when the power is off.
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To install the RAMBO board, a 74LS156 has to be removed from the Ramdisk PCB. A replacement 74LS156 is present on the RAMBO board, together with a PAL (programmable array logic) that performs all the selection operations that would otherwise require several TTL chips. The advantage is, of course, that it makes the RAMBO board smaller, but it also means that you cannot build your own board: you need a PAL programmer and you also need to know the fuse map to burn into the PAL.
Installing the RAMBO board, also implies redirecting CRU bits 1, 2 to the board. This is done by bending out these two pins and connecting them (together with the socket they were in) to the RAMBO board with thin wires. Those going to the socket holes must be connected on the back side of the board, which requires drilling a small hole in the PCB to fit them through. Nothing too difficult though. This will allow to change the value of CRU bits 1 and 2 while RAMBO mode is on, which helps preventing RAMBO pages to overlap with Ramdisk pages.
In addition, two other wires connect the RAMBO board to CRU bit 15 (RAMBO selection bit, not used by the Ramdisk) and to output Y3* of the 74LS138 address decoder (addresses >6000-7FFF, connection labelled "Rambo sel" in the schematics).
I'm not providing a schematic here. It would be pointless since all
the work is done by the PAL and I have no way to know what has been programmed
in it.
The ROS is installed in the 6264 SRAM chip that maps at >4000-57FF, and in the first SRAM page that maps at >5800-5FFF. The RAMBO-specific software, as well as any user-defined DSRs and ISRs are installed in the "hidden" part of the 6264 chip, which can be unmasked by the RAMBO board.
Provision is made for the user to create an extra DSR, with any name upto 8 characters in length. This DSR will be installed in SRAM page 1.
All regular file operations are available on these vitual drives. In addition, four extra operations are possible:
Opcode >08 , scratch record, is now valid. It can be used to remove a record in the middle of a file and shift all the following ones accordingly. It only works on FIXED files, that have previously been opened.
Opcode >0A, assembly-program, loads a "program" file containing an assembly program in EA5 format and launched it. If necessary, additional files can be loaded automatically, according to the EA5 format.
The address where the program is to start can be altered by specifying
an alternate address in the PAB, byte 2+3:
>0000 = use the address specified in the first file
>FFFF = load but don't run,
other = start at this address.
PAB bytes 6+7 can be used to specify a maximal file length. If it's >0000, the file length won't be checked.
Opcode >0B, Basic-program, enters Extended Basic, then loads and runs a TI-Basic or Extended Basic program. The file type can either be "program" or "internal, variable 254". It requires the Extended Basic cartridge since what it does is to hook the interrupt service routine, launch Extended Basic, wait till it's about to load the DSK1.LOAD file and change the name of the file for the one you specified.
Opcode >0C, cartridge, loads a "program" file containing a GRAM memory dump in your GRAM-card. It is designed to work with the Horizon P-code card, but can easily be addapted to other GRAM card.
Other differences with the TI standards include:
WO.x Write protection on, x is the drive number: 1-9 and
A-Z are valid values. The drive will behave as if it contained a write-protected
floppy.
WF.x Write protection off, x is the drive number, as above.
AO Auto-start on. This cause the ramdisk power-up routine to launch a program that can be selected with the CFG utility.
AF Auto-start off. Disables the auto-start feature (but still remember the selected program).
RD.o.n Rename drive. o is the old drive letter, n is the new one.
LD.x.name Loads a program directly (That's right, assembly in TI-Basic!). x is the drive number, "name" is the filename. This must be a "program" file that can be loaded by either opcode >0A (assembly), >0B (basic) or >0C (cartridge).
>B0 Selects a RAMBO bank. This subprogram must be called from
assembly. It actually consists in 3 subfunctions, to be distinguished with
a parameter passed in byte >834C-834D. All return an error flag in >834E-834F:
>0000 means no error, >FFFF means something went wrong (e.g. the page number
is too high, or the page is not available).
| Dir | >834A | >834C | >834E | Function |
|---|---|---|---|---|
| Call
Ret |
Current number of pages
Total number of pages |
>0000
>0000 |
-
Error flag |
Calculates the total number of RAMBO
pages for all Ramdisks in the system. |
| Call
Ret |
(not used)
CRU address of the Ramdisk |
Desired page number
CRU value to select this page |
-
Error code |
Selects the desired page, turns it on.
Also turns Gramcard and cartridges off. |
| Call
Ret |
(not used)
(unchanged) |
>FFFF
>FFFF |
-
>0000 |
Turns off RAMBO mode in all Ramdisks. |
gk.txt This one makes RAMBO mode compatible with
the german 128K Gram-Karte, instead of the P-Gram+ card.
vnb.txt This one causes the ROS to insert and
automatically update version numbers for each file. A "second best" solution
short of a time-stamp.
There are also modified versions of widely used programs, that will
accept drive numbers in the range DSKA-DSKZ (DiskU, Archiever III, etc).
