The XT: an 8088 (16-bit core, 8-bit bus) with a hard disk slot and one switch block.
| Chip | Where | Role |
|---|
| 8088 microprocessor | | CPU with a 16-bit core and an 8-bit external bus. Used in the IBM 5150, 5160, 5155, PCjr and the HERO 2000. |
| 8284A clock generator and driver | | Crystal oscillator, divide-by-3 CPU clock, ready synchronizer and reset for the 8088. |
| 8288 bus controller | | Makes the memory and I/O strobes and the bus direction for the 8088 in maximum mode. |
| 8259A programmable interrupt controller | | Eight interrupt inputs (IRQ0 to IRQ7), priority logic and the vector for the 8088. |
| 8253-5 programmable interval timer | | Three 16-bit counters: time-of-day tick, DRAM refresh pacing and the speaker tone. |
| 8255A-5 programmable peripheral interface | | Three 8-bit programmable I/O ports (A, B, C). |
| 8237A-5 DMA controller | | Four DMA channels. PC: channel 0 refreshes DRAM, channel 2 is the diskette. |
| 8087 numeric data processor | | Optional floating-point coprocessor in the empty socket beside the 8088. |
| 4164 DRAM 64K×1 | | Main RAM (8 chips = 64 KB) |
| 41256 DRAM 256K x 1 | Banks 0 and 1 on the 256-640KB board (banks 2 and 3 are 4164) | Dynamic RAM, 256K x 1 |
| 23256 mask ROM 32K x 8 | U18, the 32K x 8 ROM (XT Technical Reference 1502237, sheet 5 of 10). 1501512 (MK38036N-25) on the photographed board | 32K x 8 mask ROM in the 27256 pinout. |
| 2764-pinout 8K x 8 ROM | U19, the 8K x 8 ROM in the second 28-pin socket (Technical Reference: one socket holds 32K, the other 8K). 6359116 on the photographed board | 8K x 8 ROM or EPROM in the JEDEC 2764 pinout. |
| 24S10 / 82S129 bipolar PROM 256 x 4 | U44, RAM bank decode (XT Technical Reference 1502237, sheet 3 of 10, draws a 24S10); N82S129AN marked 6359123 on the photographed board | Memory address decoder: a 256 x 4 lookup table that turns high address bits into chip selects. |
| MC6845 CRT controller | | Makes sync, display enable, cursor and the video RAM refresh addresses. |
| Rail | Expect | Measure at |
|---|
| +5 V | 4.80 to 5.25 V (+5 / -4 percent) | xt-psu pins P9-4, P9-5, P9-6; 8088 pin 40 |
| +12 V | 11.52 to 12.60 V (+5 / -4 percent) | xt-psu pin P8-3; I/O channel B9 |
| -12 V | -10.92 to -13.20 V (+10 / -9 percent) | xt-psu pin P8-4; I/O channel B7 |
| -5 V | -4.60 to -5.50 V (+10 / -8 percent) | xt-psu pin P9-3; I/O channel B5 |
| Power Good | LOW at switch-on, HIGH after 100 to 500 ms | xt-psu pin P8-1 |