The 6502 CPU has no single register that is able to hold any C data besides char. And, for arithmetic operations, only the A register (the accumulator) can be used. Obviously, the 6502 architecture is not very well suited for C and its larger data types.
For this reason, operands are placed in an artificial “primary register”. This register consists of the A and X CPU registers if the operand is 16 bit wide (low byte in A, high byte in X). For 32 bit operands, a 16 bit zeropage location named sreg is used in addition to these CPU registers. So the high word goes into sreg and the low word goes into A/X.
If a runtime function uses more than one operand (a multiplication for example), the first operand is passed on the parameter stack, while the second one is passed in the primary register.