In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

The SX-28 and SX-48 have five areas of difference.  
I am trying to provide a brief summary for overview.  You can make a check list of these 5 areas.

They all have some affect on your programing code.  Begin by studying and programing the SX-28.
Guenther's book primarily focuses on it.

The way that RAM is laid out, there is a convienient algorhythm to initialize all the SX-28 RAM to zero; but it won't work on the SX-48.  You have to use a 'brute force' method to do the same job. [A bit more code]

EEPROM pages are split in low and high.  Subroutines should start in the Low portion as there is a 9th bit that the Call routine always reads as Zero [you have to Call, then Jump to get to the High part in a subroutine; then return from 'far'].  So subroutines must begin in the low half of each of the 4 [on the Sx-28] or the 8 [on the Sx-48] pages.

The Checklist.

1. Ram -  The SX-48 is double the SX-28.  Both use $00 for 'global RAM
The Sx-28 uses every other bank $01, $03, $05; while the SX-48 uses All the banks $01,$02,$03....

2. EEPROM - The SX-48 has double the SX -28.  
The Sx-28 has 4 pages; the SX-48 has 8 pages

3. Mode -  The SX-48 has more Modes due to additional timers so the code is not quite the same

4. Ports - The Additional D and E ports use space that in the SX-28 may be used for global RAM in the SX-28

5. Additional timers -  Compare and capture features in hardware may eliminate the need for some software timers.

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