[jezze]

I think it would have been better if they had designed it so that the error message from the kernel came in a seperate register. That would mean you didnt have to use signed int for the return value. The issue is that one register now is sort of disambigious. It either returns the thing you want or the error but these are seperate types. If you had them in seperate registers you would have the natural type of the thing you are interested in without having to convert it. This would however force you to first check the value in the error register before using the value in the return register but that makes more sense to me than the opposite.

[bhawks]

A whole separate register?

That is quite expensive. Obviously you need to physically add the register to the chip.

After that the real work comes. You need to change your ISA to make the register addressible by machine code. Pdp11 had 8 general purpose registers so they used 3 bits everywhere to address the registers. Now we need 4 sometimes. Many op codes can work on 2 registers, so we need to use 8 out of 16 bits to address both where before we only needed 6. Also pdp11 had fixed 16 bits for instruction encoding so either we change it to 18 bit instructions or do more radical changes on the ISA.

This quickly spirals into significant amounts of work versus encoding results and error values into the same register.

Classic worse is better example.

[dwattttt]

> A whole separate register?

There are quite a few registers (in all the ISAs I'm familiar with) that are defined as not preserved across calls; kernels already have to wipe them in order to avoid leaking kernel-specific data to userland, one of them could easily hold additional information.

EDIT: additionally, it's been a long time since the register names we're familiar with in an ISA actually matched the physical registers in a chip.

[jezze]

Yeah I am not advocating creating a new seperate register, even though that would be nice. Like the poster below said, there are usually some unpreserved registers to choose from but if you for some reason cant spare a register you could instead write the error code to any virtual address instead, or send a signal, a message or anything else you could come up with. Just some way that does away with this intermix of return types and error types.