analog_programmer wrote on 2024-04-06, 12:43:There are two "A413256J06" SOJ 26-pin chips soldered directly on the card's PCB paired with two socketed DIP-20 Samsung KM44C256 […]
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mkarcher wrote on 2024-04-06, 11:53:
Some parts of the chip number "A413256J06" have a meaning obvious to experienced electronics engineers: "256" is either the amount of kilobits or the depth (i.e. the number of addresses). On a 512KB card, I expect 4 chips of 256K x 4. "J" is the package type SOJ. "06" is the access time: "60ns". The "3" in the type code feels kind of unusual, but might just be a vendor-specific indication of fast page mode support. If the chips are in a SOJ26/20 package (like the parity chips you need to complete the 4M modules), they are extremely likely to be standard 5V FPM 256K x 4 chips, commonly called 44256. Those were by far the most common memory chips used on entry-level 1MB SVGA cards around 1990-1994. The cross reference I posted should mention different vendor-specific chip type numbers for that kind of memory.
There are two "A413256J06" SOJ 26-pin chips soldered directly on the card's PCB paired with two socketed DIP-20 Samsung KM44C256AP-10 (256K x 4) for a total of 512 kB VRAM, so I think you're absolutely right about the chip's size, voltage and speed. I can add two more DIP-20 (in sockets) or SOJ26 chips soldered on the PCB.
This is the very same VGA card: https://www.vgamuseum.info/images/palcal/umc/ … 08af_top_hq.jpg (on this picture the socketed chips are missing)
There are memory configurations with six VRAM chips on the same videocard model: https://www.vgamuseum.info/index.php/cards/it … -umc-um85c408af
I'm afraid that the situation is likely more complicated, and you are unable to exceed 512KB on that type of card, although you are perfectly correct that there are cards with six chips shown at the VGA museum. Looking at https://www.vgamuseum.info/images/zaatharen/u … 5C408AF_fhq.jpg and https://www.vgamuseum.info/images/palcal/umc/ … d03a_top_hq.jpg with 4 chips, you will find chips labeled A414256J (I'm going to ignore the speed grade 07 or 06), while a card with 6 chips like https://www.vgamuseum.info/index.php/cards/it … -umc-um85c408af has A413256J chips on them.
My current speculation is that the 4 vs. 3 in the chip type actually represents 256K x 4 and 256K x 3! The A413256J chips I see at the VGA museum are likely partially broken chips where one of the four bits is not used. If that idea is correct, the 4-chip cards have two rows of two chips each, providing 2*4 = 8 bits per row. The 6-chip cards have two rows of three chips each, but each chip only contributes 3 bits, so 9 bits per row. The soldered jumpers MJP1-MJP6 can be used to select what RAM data pins are connected to what chip data pins. On the photo of the 6-chip cards, the data traces from the chip to the MJP solder bridges are completely at the top layer, so it's easy to see that each of the 8 jumpers connects to one pin on the chip. In the 6-chip configuration, the chip-connected solder positions are the top one of each individual jumper wire. In addition to the 8 solder positions connected to the UMC VGA chip, there are 8 used and 4 unused solder positions connecting to the RAM. The RAM-side connections of the jumper pins are on the bottom layer of the card.
So what this essentially means: Each row (of two or three chips) can physically provide 12 data bits (4 bits routed to each memory chip), 8 of those 12 bits are jumper-wired to the VGA chip and 4 of those 12 bits are ignored. If your card operates properly with just 2 chips per row, it means all 8 bits that are physically present actually work. So I am slightly surprised that your card has chips labelled A413256J instead of A414256J. Possibly the graphics card manufacturer tested the chips themself and found out that all 4 bits in this chip work well enough. As already all 8 bits per row are populated, there is no point in adding more RAM chips. So I expect that keeping to the current slogan in the topic "let's not make stupid things again", you don't try to upgrade that card.
