The Sinclair ZX81 has 8K Basic interpreter ROM; the Spectrum has 16K. What extra features does the Spectrum put in the extra 8K?

The most obvious thing is commands for color graphics, which are absent from the ZX81 partly to save space, and mostly for the excellent and sufficient reason that the machine has no color graphics. However, it seems unlikely that the entire extra 8K, fully half the Spectrum ROM, is dedicated entirely to color graphics.

There is a BEEP command, though this seems unlikely to take much space.

The Spectrum optimizes arithmetic when the numbers happen to be integers; according to Is there any significant difference in Sinclair BASIC of ZX80, ZX81 and ZX Spectrum? the ZX81 uses the floating-point code path in all cases. Presumably this spends some space in return for speed.

The Spectrum tape I/O is faster. Does this code take up more space?

The Spectrum has about 1K of unused ROM space, due to development schedule pressure.

What else does the Spectrum use the extra ROM for?

The Sinclair ZX81 has 8K BASIC interpreter ROM; the Spectrum has 16K. What extra features does the Spectrum put in the extra 8K?

The most obvious thing is commands for color graphics, which are absent from the ZX81 partly to save space, and mostly for the excellent and sufficient reason that the machine has no color graphics. However, it seems unlikely that the entire extra 8K, fully half the Spectrum ROM, is dedicated entirely to color graphics.

There is a BEEP command, though this seems unlikely to take much space.

The Spectrum optimizes arithmetic when the numbers happen to be integers; according to Is there any significant difference in Sinclair BASIC of ZX80, ZX81 and ZX Spectrum? the ZX81 uses the floating-point code path in all cases. Presumably this spends some space in return for speed.

The Spectrum tape I/O is faster. Does this code take up more space?

The Spectrum has about 1K of unused ROM space, due to development schedule pressure.

What else does the Spectrum use the extra ROM for?