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# The ARM1 processor’s flags, reverse engineered

## The conditions flags in detail

The flags all roughly follow the circuit described above, but there are differences since the flags have different behaviors.
The schematic below shows the circuits for the four condition flags: V, C, Z and N.
This section describes these flags in detail, along with how the control signals are generated. By comparing the chip logic with the documentation, we can see how the described behavior
is implemented in the logic.

### Generating the flags

Each flag is generated in a different way.
The N (negative) flag is very simple. A signed number is negative if the top bit is set, so the N flag is simply loaded from the top bit of the ALU bus.

The Z (zero) flag is generated by the ALU. The ALU in effect does a NOR of all 32 output bits; if all bits are zero, the Z flag is 1. For efficiency, the ALU uses a chain of alternating NAND and NOR gates, but the effect is the same.

Generating the C (carry) flag is quite complicated. For arithmetic operations, the carry flag is the carry out from bit 31 of the ALU: this is the carry for addition and not-borrow for subtraction. The ARM1 supports a variety of shift operations, which affect the carry in different ways, so logic gates select different bits from the shifter depending on the instruction. It may be the bit shifted out on the left, the bit shifted out on the right, the carry flag, the left bit or the right bit.

The V (overflow) flag indicates overflow of a signed value. If two signed values are added or subtracted, the result may not fit in 32 bits, and this is indicated by setting the overflow flag. An overflow occurs if the carry out from bit 30 being different from the carry out from bit 31 and is computed by XOR of these two bits. I discuss signed overflow in detail here.

Schematic of the condition flags in the ARM1 processor: OVerflow, Carry, Zero, and Negative.

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