SuperAMS (Super Asgard Memory System, a.k.a. SAMS card) was one of the first memory expansion card designed for the TI-99/4A. It was designed around 1993 by Asgard Inc, under the managment of Jim Krych. Its elegant simplicity is due to the fact that it's build around the 74LS612 memory mapper, once commonly used in PCs and in Nintendo game cartridges. For some reason, the card never got the popularity that it deserved: I was told that only about 100 were produced. Originally, this may have been due to the high cost of SRAM chips: the card was designed to accomodate upto 1 megabyte, which was quite expensive in the early nineties. Nowadays, the problem is rather with the 74LS612 that became extremely hard to find: most PCs now emulate it in a VLSI chip.
The 74LS612 memory mapper
Theory of operation
AEMS: the ancestor
16 megs modification
This very handy TTL chip serves to extend an address bus by 8 extra bits. It does this by storing 12-bit values into 16 internal registers. When mapping is enabled, four lines from the address bus serve to select a register and the content of the register is transfered to 12 output address lines. In effect, the 74LS612 expands 4 lines into 12, which multiplies the address space by 256. For the TI-99/4A, this means that the maximum memory becomes 16 megabytes.
To access all the extra memory, the CPU must load selected values into the proper register. Typically, the address space will be divided in 16 banks, and 256 different "pages" can be selected to appear in these banks. The size of a bank is that of the address space divided by 16 (8K on the TI-99/4A).
Of course, the TI-99/4A does not support RAM in the whole address space, only at >2000-3FFF and >A000-FFFF. This means that only 8 registers will effectively be used.
The memory mapper comes in four flavors: the 74LS612 described here, the 74LS610 which has the ability to latch the address bus, the 74LS611 and 74LS613 which are open-collector versions of the above. This means that, if you cannot find a 74LS612, you can go for any of the others. You just need to make sure pin 28 will be pulled up to +5V with the '610 and '611. If you're using an open-collector chip, you will need extra pull-up resistors on the outputs MO0-MO11 (4.7 KOhms to +5V should be ok).
RS2 |1 o 40| Vcc
MA3 |2 39| MA2
RS3 |3 7 38| RS1
CS* |4 4 37| MA1
STROBE* |5 L 36| RS0
R/W* |6 S 35| MA0
D0 |7 6 34| D11
D1 |8 1 33| D10
D2 |9 2 32| D9
D3 |10 31| D8
D4 |11 30| D7
D5 |12 29| D6
MM* |13 28| nc
MO0 |14 27| MO11
MO1 |15 26| MO10
MO2 |16 25| MO9
MO3 |17 24| MO8
MO4 |18 23| MO7
MO5 |19 22| MO6
GND |20 21| ME*
Vcc: +5 Volts
CS*: Chip select. When active (low) the CPU can access the mappers registers via D0-D11.
R/S*: Direction: high to read from registers, low to write to registers.
STROBE*: Write impulsion: stores D0-D11 into a mapper register (CS* and R/W* must be low).
RS0-RS3: Register select. Address of the register to access.
MM*: Map mode. When active (low) MO0-MO11 reflect the content of the register selected by MA0-MA3. When high, MO8-MO11 reflect MA0-MA3, and MO0-MO7 are low ("transparent" mode).
MA0-MA3: Map address. Four lines of the memory address bus that selects a register in the mapper (MM* low).
MO0-MO11: Map output. Twelve new address lines, corresponding to the content of the register selected by MA0-MA3 when MM* is low.
ME*: Map enable. A low level activates MO0-MO11. When high, MO0-MO11 are in high impedance state (isolated).
C: Not connected on the 74LS612/74LS613. On the 74LS610/74LS611: latch control. When high the content of the selected register is transparently passed to MO0-MO11. A low level latches the outputs and freezes MO0-MO11, even if the register (or its content) changes.
The 74LS612 supports four modes of operation: read, write, map and pass.
|CS*||MM*||R/W*||Mode||Register selection||MO0 - MO11 outputs|
|Low||x||High||Read||RS0-RS3||(Depend on MM* and ME*)|
|Low||x||Low||Write||RS0-RS3||(Depend on MM* and ME*)|
|High||Low||x||Map||MA0-MA3||Reflect register contents|
|High||High||x||Pass||none||MO8-MO11 = MA0-MA3
To load a value into a register, the CPU places the address of this register on RS0-RS3, and sets CS* and R/W* low. A low pulse on STROBE* performs the write operation and causes the data present on D0-D11 to be stored into the selected register.
The content of a register can be read back by setting CS* low and keeping R/W* high. The data contained in the register selected by RS0-RS3 will be placed on D0-D11.
To perform memory mapping, the CPU sets CS* high and MM* low. In this configuration, the 74LS612 ignores the inputs RS0-RS3 and uses MA0-MA3 instead to select a register. The content of this register is presented on MO0-MO11.
To disable memory mapping, the CPU can set both CS* and MM* high. In this case, the 74LS612 is "transparent", i.e. forwards RA0-RA3 to the MO8-MO11 pins. The other MO pins are set low.
Two additional control pins are provided: ME* serves to place the output pins MO0-MO11 in high impedance state, when the mapper should not be in control of the address bus. On the 74LS610/611, the additional C pin allows to freeze the outputs in a given state. This may be usefull if the CPU needs to alter the register contents or to place "crazy" values on MA0-MA3.
|\_____________________________________/ \_____________________ CS*
| >20 ns | | >20 ns | |
| |\ | | /| | | R/W*
| | >20| | >20 | | |
| | |\ >75 /| | | | STROBE*
| ||>20| |>20| | | |
XXXXXXXXXXXXXXXX Address valid X Address valid X New address RS0-RS3
|26-50| |30-50| | >75 |>20 | |20-35| |38-65 | |26-50| |39-75|
-----------X output X--X input X-------X output X--------X output X------ D0-D11
____|__48-85 ns |
|\ | /| MM*
|22-40 ns| |
\ | | /14-25| ME*
| | | |
XXXXXX address valid X address valid X valid XXXXXXXXXXXXXX MA0-MA3
| | 39-70 ns | | | |
|17-30| | |22-40| |13-30| |
--------X valid X valid X valid X valid X valid X------- MO8-11
| | | | |
--------X valid X valid X valid \ ,------- MO0-7
| MAP | PASS
Supply voltage: Vcc 7V
Input voltage: D0-D11 5.5V
All others 7V
Free air temperature: 0 to 70 `C
Storage temperature: -65 to 150 `C
|Supply voltage, Vcc||4.75||5||5.25||V|
|High-level input voltage||2||.||.||V|
|Low-level input voltage||.||.||0.8||V|
|High-level output current MO0-MO11
|Low-level output current MO0-MO11
|High-level output voltage MO0-MO11
|Vcc = 4.75V I =-3mA
Vcc = 4.75V I = max
|Low-level output voltage MO0-MO11
|Vcc = 4.75V I = 12 mA
Vcc = 4.75V I = 24 mA
Vcc = 4.75V I = 4 mA
Vcc = 4.75V I = 8 mA
|High-level Z output current||Vcc = 5.25V Vout = 2.7V||.||.||20||uA|
|Low-level Z output current MO0-MO11
|Vcc = 5.25V Vout = 0.4V||.||.||-20
|Input current D0-D11
all other inputs
|Vcc = 5.25V Vin = 5.5V
Vcc = 5.25V Vin = 7V
|High-level input current||Vcc = 5.25V Vin = 2.7V||.||.||20||uA|
|Low-level input current||Vcc = 5.25V Vin = 0.4V||.||.||-0.4||mA|
|Shortcut output current MO0-MO11
|Vcc = 5.25V 1 pin 1 sec||-40
|Supply current||Vcc = 5.25V Outputs high
Apart for the mapper and SRAM memory chips, the SuperAMS card contains few additional circuitery.
Nothing special here: a LM7805CT voltage regulator and two caps (suggested values are mine: there are none on the shematics I got). In addition, it's always a good idea to add a by-pass cap in front of each chip on the card, to filter out transients.
As recommended by Texas Instruments, all lines to/from the peripheral bus are buffered on-card. This is done with three 74LS244 for the address bus (16 bits) and some of the control lines: MEMEN*, CRUCLK, RESET*, DBIN, and WE*.
The data bus, of course needs a bidirectional 74LS245 buffer. The DIRection is controlled by the DBIN signal from the peripheral bus, the ENable pin by the CardSel* signal generated by the card (see below).
The CardSel* signal is also connected via a 74LS125 to the RDBENA* line of the peripheral bus to enable the drivers in the connection card and the console-end of the cable. A 74LS125 is necessary, so as not to hold the line high when we are not using it, as another card may need it. The CardSel* signal enables the '125 which input is connected to the ground, the rest of the time the 74LS125 is in high-impedance (i.e. isolated) state.
The following schematic combines the above buffering circuits and the CRU logic described in the next chapter.
The CRU logic is very basic. A 74LS688 comparator is used to compare the CRU address to a hardwired value of >1E00 (possible improvement on the design: use a DIP switch on B4-B7). The output of the comparator enables latching of the A15/CRUOUT bit by the 74LS259 addressable latch. Lines A12-A14 are used to select a bit among eight. Note that no provision is made to read back the status of the bits. Anyhow only two bits are used, after due inversion by 74LS04 inverters.
Addresses are decoded by a 74LS138 3-to-8 decoder that splits the addressing space into eight >2000-byte chunks. The active low outputs Y1*, Y5*, Y6* and Y7* are combined with AND gates to produce an active low signal, MemExp*, upon accesses in the range >2000-3FFF (Y1*) or >A000-FFFF, i.e. the usual memory expansion space.
In addition, Y2 is combined with Bit0* to access the memory mapper registers in the address block >4000-5FFF, when CRU bit 0 is set as 1. Both this signal and MemExp* are combined in a fourth AND gate, to yield the CardSel* signal that was discussed above.
The SRAM data pins D0-D7 are connected to the data bus lines D7-D0, in reverse order because TI numbers the lines the opposite way to everyone else. The address pins A0-A11 are connected to the address lines A4-A15, providing an >1000-byte (4K) address bank. The extra address pins A12-A18 serve to select the RAM page that should appear in this bank. They are controlled by the output pins of the 74LS612 memory mapper, M5-M11.
In addition, two jumpers provide the possibility to use either one or two 512 Kbyte or 128Kbyte SRAM chips. The MemExp* signal that should normally go directly to the CS* pin of the SRAM is filtered through two OR gates. One receives its input directly from the first jumper, the other receives the inverse of this signal. The jumper serves to select either MO4 or MO6 as the line that controls which SRAM chip is accessed. The other jumper is connected to A17 on both chips and offers the choice between MO6 or +5 volts: when using 128Kbytes chips (that have less pins) the jumper should be set on +5 volts.
This gives us two possible configurations:
Note that it is possible to install only one chip, so the card can have four possible memory sizes: 128K, 256K, 512K or 1024K. Just be carefull not to select a page that maps into the missing chip! There is no such problem when using 128K instead of 512K chips because higher page numbers just mirror lower pages, e.g. pages 64, 128 and 192 are the same as page 0.
The R/W* pin is connected to the WE* line and the OE* pin to the inverse of DBIN (but could probably be grounded instead).
Address lines A11-A14 are connected to the RS0-RS3 pins: they select one of the 16 internal registers of the 74LS612 when the software accesses the registers directly (address range >4000-4020). Each register is 12 bits wide but only 8 bits are used: the most significant pins D4-D11 are connected to the data bus lines D0-D7. The remaining pins D0-D4 are grounded. This allows the software to load a value from 0 to 255 into any of the registers.
When the 74LS612 is used as a mapper it chops the address space in 16 chunks of >1000 bytes and provides a 10-bit value, taken from the appropriate register, on MO0-MO11. Only the most significant 8 bits are used and serve to select a SRAM page. Address lines A0-A3, connected to the MA0-MA3 pins, select the appropriate register while in mapping mode.
Pin R/W* is controlled by DBIN, pin STB by WE*, and pin CS* by the MapSel* signal (address >4000-5000, with CRU bit 0 set to 1), while the ME pin receives the inverse of the latter signal. Pin MM enables memory mapping when an access occurs in the range >2000-3FFF or >A000-FFFF and CRU bit 1 is set as '1'. This CRU bit can also be reset as '0' to put the mapper in "transparent" mode. In this mode, >2xxx maps to page >0002, >3xxx maps to page >0003, >Axxx maps to page >000A, etc. Note that CRU bits are reset upon power-up, so this mode will be the default. The software can then quietly load other values into the registers, then set bit 1 to '1' and the new mapping will come into effect.
Before the SuperAMS, came AEMS (Asgard Extended Memory System, if I'm not mistaken). It suffered from several limitations, or so I was told:
But then again, I didn't see the schematics for this one...
A possible improvement to the design would be to make use of the extra mapping lines M0 through M3. This could be done easily with a 74LS273 latch, controlled by A15 and WE* so as to memorize the odd-address byte upon any write operation (the odd byte is always written first). Four of the input lines would be connected to the D4-D7 data bus lines, and the corresponding outputs to the D0-D3 pins of the 74LS612. This way, page numbers from 0 to 4096 can be stored into the registers. The output MO0-MO4 could drive two 74LS154 decoders and select a SRAM chip among 32, bringing the total available memory to 16 megabytes, arranged as 4096 pages of 4 Kbytes each.
Alternatively, only one 74LS154 decoder could be used which would allow only 16 chips, i.e. 8 megabytes. In this case, MO0 could control the second enabling pin of the decoder. This would let us turn mapping off on a per-bank basis. The big advantage of such a design would be to enable mapping in the area >4000-5FFF (card DSRs, we'll need another CRU bit to select SRAM or 74LS612 in this area) and >6000-7FFF (cartridge banks and RAMBO banks). This cannot be done with the current design because mapping can only be disabled globally, not for given memory windows.
Here is a tentative schematic:
The 16 megs version is identical except that there will be two 74LS154, both with their G1* input connected to MemExp*. One of them should have its G2* input connected directly to MO0 and the other should receive MO0 inverted by a 74LS05 on its G2* input. Obviously, each decoder will drive a different set of 16 SRAM chips.
Finally, here is a possible modification of the address decoder that only requires one extra TTL chip (74LS30):
74LS138 4.7K 74LS30
Y1*, Y5*, Y6* and Y7* are combined via an 8-inputs NAND gate and trigger the active-low MemExp* signal when an address in the range >2000-3FFF or >A000-FFFF is accessed. Y3* plays the same role for addresses in the range >6000-7FFF.
Y2* is combined via an OR gate with the inverse of CRU bit 0, so that addresses in the range >4000-5FFF are only active when CRU bit 0 is set to 1. This signal is further ORed with CRU bit 2 or its inverse so that when bit 2 is 0 the 74LS612 maps at >4000-5FFF, whereas when bit 2 is set to 1 the SRAM maps in this area.
The switches and corresponding pull-up/pull-down resistors are optional: they let you disable the extra mapping areas and have the card react just like the original Superam (only with more memory). S1 disables mapping at >6000-7FFF when open. S2 disables CRU bit 2 when open, so >4000-5FFF always accesses the 74LS612 registers. Note that this can be done by software on the 8 mega card, since MO0 is used to enable/disable mapping.
You will have noted that the original SuperAMS does not map any memory at >4000-5FFF, thus cannot contain any DSR. This is because it is a memory expansion card, and these do not need DSRs.
However, we still need some way to control memory paging. This must be done by the application program.
The principle is very easy. You can set a page number by setting CRU bit 0 to 1, and writing a page number into 8 among the 16 available registers.
It may seem harmless to store arbitrary values in the remaining registers, as mapping never occurs in the area that they control. However, these should not be used to store data irrelevant to mapping, as a future version of the card may want to extend mapping to the >4000-7FFF area, for instance.
As mentionned above, CRU bit 1 can be used to toggle between mapping and transparent mode. A SBO 1 enables mapping in the whole area >2000-3FFF and >A000-FFFF, a SBZ 1 puts it in transparent mode (page 2 at >2000-2FFF, page 3 at >3000-3FFF, etc)..
* Set page number for a memory bank
* Enables mapping
MAPOFF LI R12,>1E00 Superams CRU address
* Set page number, for 8 Megs modification
Note that the old version of MAPAGE also works with an 8 megs card .
The MOVB instruction will latch >FF in the 74LS273 because it first
reads the odd byte to replace it later, and the card does not answer so
all lines read high. The old software will thus access the last SRAM
And thanks to the inverter between 74LS273/Q0 and 74LS612/D0, mapping
always be on.
A complete development system (assembler, linker, etc) was released by R.A. Green of RAG Software, that lets you write assembly programs in a transparent manner, i.e. without having to worry about page switching.
A variant of C99 was designed by Joe Delekto to take advantage of the SuperAMS card.
Brad Snyder created "XBpacker", an utility that allows to store several Extended Basic programs in the SuperAMS card, or even one huge XB program written in a modular fashion.
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