Working on the 2121 support in IBMulator, I started looking for any kind of information about the video interface.
According to IBM Corp., the PS/1 2121 has got a standard VGA with 256KB of memory, very much like the 2011’s one.
Actually, the chip used is the IBM 84F7985 / TI CF62011BPC. Unfortunately there’s no datasheet and information is very scarce (if you have any technical info please let me know!)
U20: IBM 84F7985 / TI CF62011BPC
U36: INMOS IMSG171P-35, 35MHz color LUT
U24: Toshiba TC511665BJ-10, 100ns 65K x 16-bit (128KB) CMOS DRAM
U35: Toshiba TC511665BJ-10, 100ns 65K x 16-bit (128KB) CMOS DRAM
U21: unpopulated DRAM solder pads
U29: unpopulated DRAM solder pads
The TI CF62011BPC is a SVGA chip and it could use up to 1MB of memory, but on the PS/1 2121 was configured with only 256KB, limiting it to the standard VGA modes.
It was also used on other IBM Personal System machines of the same era, for example the PS/2 Model 25SX. This means that VESA DOS support can be obtained using the same TSR driver: VESA.EXE
Depending on the amount of VRAM installed, the VESA.EXE driver adds support for these additional video modes:
- 640×400 256-color graphics
- 640×480 256-color graphics
- 132-column x 25-row text
According to www.walshcomptech.com, it’s the same adapter as the IBM SVGA Adapter/A (MCA ID 090EEh), with the most notable differences being faster VRAM and RAMDAC. The VESA DOS graphics driver for that card can reportedly be used: m95svga.exe
It responds at ports in the range 0x2100-0x210F, which are the same as the XGA.
No drivers for Windows are known to exist, but OS/2 seems to have a driver for it.
PS/1 enthusiast DistWave managed to solder a couple of memory sockets and expand the VRAM to 512KB, making it possible to select the 640x480x256 video mode using the VESA.EXE driver for the PS/2 25SX.
A usenet message in the comp.sys.ibm.ps2.hardware group dated 21 Dec 2003 shed some light on the origin of the IBM SVGA/A (90EE) adapter:
In the early 90s IBM looked for some cheap VGA card to substitute the
(relatively) expensive XGA-2 adapter for *servers*, where the primary purpose
is supervision of the machine rather than real *work* with it in Hi-Res. It was
just to supply a base video, where a XGA-2 were a waste of potential. They had
a contract with TI for some DSPs in multimedia already (the MWave for instance
is based on TI-DSPs as well as many Thinkpad internal chipsets) and TI offered
them a rather cheap – and inexpensive – chipset and combined it with a cheap
clock oscillator and an Inmos RAMDAC. That chipset was already pretty much
outdated at that time but IBM decided it would suffice for that low end
Driver support was given under DOS and OS/2 only for base functions like
selection of the vertical refresh and few different modes only. Not even the
Win 3.x support has been finalized. Technically the adapter could do better
than VGA, but its video BIOS is largely undocumented and intentionally crippled
down to a few functions.