The primary register

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.

cc65/the_primary_register.txt · Last modified: 2010-05-01 19:26 by uz
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