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3-9

ADDRESS SPACES

NOTE

If your program executes exclusively from on-chip ROM/OTPROM/EPROM

(not from external memory), beware of executing code from the upper eight

bytes of the on-chip ROM/OTPROM/EPROM (FF:1FF8H–FF:1FFFH for 8

Kbytes, FF:3FF8H–FF:3FFFH for 16 Kbytes). Because of its pipeline

capability, the 8XC251Sx may attempt to prefetch code from external memory

(at an address above FF:1FFFH/FF:3FFFH) and thereby disrupt I/O ports 0

and 2. Fetching code constants from these eight bytes does not affect ports 0

and 2.

If your program executes from both on-chip ROM/OTPROM/EPROM and

external memory, your code can be placed in the upper eight bytes of the on-

chip ROM/OTPROM/EPROM. As the 8XC251Sx fetches bytes above the top

address in the on-chip ROM/OTPROM/EPROM, the code fetches automati-

cally become external bus cycles. In other words, the rollover from on-chip

ROM/OTPROM/EPROM to external code memory is transparent to the user.

3.2.2.1 Accessing On-chip Code Memory in Region 00:

The 87C251SB, SQ and the 83C251SB, SQ can be configured so that the upper half of the 16-

Kbyte on-chip code memory can also be read as data at locations in the top of region 00: (see

“Configuration Bytes” on page 14-7). That is, locations FF:2000H–FF:3FFFH can also be access-

ed at locations 00:E000H–00:FFFFH. This is useful for accessing code constants stored in

ROM/OTPROM/EPROM. Note, however, that all of the following three conditions must hold for

this mapping to be effective:

• The device is configured with EMAP# = 0 in the UCONFIG1 register (See Chapter 4).

• EA# = 1.

• The access to this area of region 00: is a data read, not a code fetch.

If one or more of these conditions do not hold, accesses to the locations in region 00: are referred

to external memory.

NOTE

Remapping does not apply to the 87C251SA, SP and the 83C251SA, SP.

Table 3-2. Minimum Times to Fetch Two Bytes of Code

Type of Code Memory State Times

On-chip Code Memory 1

External Memory (page mode) 2

External Memory (nonpage mode) 4

Intel 8XC251SA User Manual

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