The 68HC05 is a Von Neumann computer. All storage and input/output resources are memory mapped into a single linear address space.
The memory map is organized as bytes, begins at $0000, and ends at $xxFF, where, depending on factors like ROM or EPROM array size, 'xx' is $03, $07, $0F, $1F, $3F, or $7F.
The address space on a 68HC05 device is usually sized just large enough to contain the integrated ROM or EPROM, RAM, and control registers. The MC68HC705KJ1, for example, has a 2-Kbyte ('xx' equals $07) memory map containing 1240 bytes of EPROM, 64 bytes of RAM, and 14 bytes of other processor accessible resources.
The 68HC05 is also a Big Endian machine. A 16-bit piece of data, or word, is stored in memory with its high byte at address N and its low byte at address N + 1. This ordering applies whether the word is part of an assembled instruction (such as an index register offset), a return address residing on the stack, or the address of a service routine contained in an interrupt vector. See the examples above.