"Figure 57L: Vertical Sync Generation. The ULA contains a nine-bit vertical line counter. Upon reaching line 312 (PAL standard), the reset logic triggers a vertical blanking period. Designers must ensure that the CPU’s NMI (Non-Maskable Interrupt) is asserted exactly during the flyback period to permit tape loading routines."
This synchronization is why Spectrum tape loading was so robust (and so loud!). The ULA generated an interrupt every 20ms. The ROM code counted these interrupts to measure the length of tape pulses.
The ULA generates the master clock signal (typically 14 MHz) and divides it down to create the CPU clock (3.5 MHz). The book explains the specific timing tolerances and why the Spectrum's CPU speed was slightly variable between different motherboard revisions.
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To understand the keyword, you must first understand the component. The Zx Spectrum Ula How To Design A Microcomputer Pdf 57l
In a typical 1980s computer, you would find separate chips for:
The ZX Spectrum ULA (part number 5C112E, later 6C001E-7) combined all of these into a single 40-pin DIP package. It was the conductor of an orchestra of discrete parts: the Z80 CPU, the 16K or 48K of DRAM, the ROM, and the cassette interface.
Key functions of the ULA:
Without the ULA, the Spectrum would be a box of loose wires. With the ULA, it became a £125 miracle. "Figure 57L: Vertical Sync Generation
