The Portable: an 8088 (16-bit core, 8-bit bus) on the XT board, in a carry case.
| 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 | U1 (XT Technical Reference 1502237, sheet 1 of 10; same 64-256KB board) | 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. |
| MC6845 CRT controller | | Makes sync, display enable, cursor and the video RAM refresh addresses. |
| 4164 DRAM 64K×1 | | Main RAM (8 chips = 64 KB) |
| 41256 DRAM 256K x 1 | Banks 0 and 1 on the photographed board: HY53C256LS-10 (CMOS 256K x 1), a later upgrade; the 64-256KB board takes 4164s in all four banks | Dynamic RAM, 256K x 1 |
| 23256 mask ROM 32K x 8 | U18, the 32K x 8 ROM (as on the XT). T1501512 (TMM23256P) 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 (as on the XT). 6359116 on the photographed board | 8K x 8 ROM or EPROM in the JEDEC 2764 pinout. |
| 24S10 / 82S129 bipolar PROM 256 x 4 | U44 (same 64-256KB board as the XT); the marking is not legible on the photographed board | Memory address decoder: a 256 x 4 lookup table that turns high address bits into chip selects. |