The US version of the ROM (based on the regular UK 1.20 ROM) has differences for the American TV standard. These changes make almost all existing software entirely incompatible. Any games or software that writes directly to the screen will fail due to the different screen sizes requiring different memory addresses. Even 'legal' basic programs will likely fail due to the differing vertical text and graphics resolution. The MODE independent graphics coordinate system used across all graphics MODEs in the UK MOS is 1280 x 1024, but in this US version it is 1280 x 800. If these issues were to be overcome then the screen refreshes at 60Hz instead of PAL's 50Hz, which also would have affected the speed of many games.

Changes from MOS 1.20 to 1.20us:

• ● Code to support 16K has been removed. The US BBC Micro is 32K only.
• ● Boot message is 'Acorn OS' (without any '32K' suffix).
• ● OSBYTE 0 and *HELP both report 'OS A1.0' (instead of 'OS 1.20')
• ● OSBYTE 129 with X=0, Y=255 returns with X=$FE, Y=0 (instead of the usual X=$FF,Y=$FF) as a method for programs to identify the US model.
• ● OSBYTE 240 indicates country code (1 = US, 0 = UK)
• ● Vertical adjust (*TV) defaults to 1
• ● The interlace flag defaults to ON and is preserved over a soft BREAK (interlace flag memory position is swapped with vertical adjust variable).
• ● MODEs have fewer character rows due to NTSC standard: - MODES 0,1,2,4,5 have 25 character rows instead of 32. - MODE 3,6 have 22 rows instead of 25. - MODE 7 has 20 rows instead of 25.
• ● MODEs 0,1,2,3 start at memory address $4000, MODE 4,5 start at $6000 and MODE 7 stays at $7C00
• ● There are 200 pixels vertically in all graphics MODEs, instead of the regular 256.
• ● The standard coordinate system changes from (1280 x 1024) to (1280 x 800)
• ● The unrolled screen clearing routine takes up less memory (saving 48 bytes) due to the reduced resolution.
• ● Error message 'File not found' is now 'Not found' (to save 5 bytes of precious memory?)
• ● Minor bug fixes.