Z8
®
CPU
User Manual
UM001604-0108 Address Space
19
Program Memory
The first 12 bytes of Program Memory are reserved for the interrupt vectors, as displayed
in Figure 8 on page 20. These locations contain six 16-bit vectors that correspond to the
six available interrupts. Address 12 up to the maximum ROM address consists of on-chip
mask-programmable ROM. Refer to the product data sheet for the exact program, data,
register memory size, and address range available. At addresses outside the internal ROM,
the Z8
®
CPU executes external Program Memory fetches through Port 0 and Port 1 in
Address/Data mode for devices with Port 0 and Port 1 featured. Otherwise, the program
counter continues to execute NOPs up to address
FFFFh, roll over to 0000h, and continue
to fetch executable code (see Figure 8).
The internal Program Memory is one-time programmable (OTP) or mask programmable
dependent on the specific device. A ROM protect feature prevents dumping of the ROM
contents by inhibiting execution of the LDC, LDCI, LDE, and LDEI instructions to Pro-
gram Memory in all modes. ROM look-up tables cannot be used with this feature.
The ROM Protect option is mask-programmable, to be selected when the ROM code is
submitted. For the OTP ROM, the ROM Protect option is an OTP programming option.
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