Undocumented 16-bit I/O addressing on Z80

Question

I notice from the Zilog datasheet on the Z80 that with the I/O (IN and OUT) group of instructions, the contents of various registers are often placed in the top 8 bits of the address bus (depending on the instruction), with the lower 8 bits selecting one of up to 256 theoretically connected devices.

My question is what is the point of doing this with these upper 8 bits? I know some machines use this in someway related to decreasing decoding complexity, but are they seriously used for anything? I want to implement the instructions exactly as the Z80 suggests, but I don't see the point in implementing this behaviour as it is non-standard. This behaviour is described as undocumented, so on a 'Sega Master System' for example, will I get away with this? Many thanks.

Regards, Phil Potter

Oxcart

Undocumented but on datasheet? – Gwinn

Sorry what I meant is although this behaviour is normal - the addressing of I/O devices is offically only 8-bit. – Oxcart

Answer 1

The behavior is fully documented by Zilog (pages 269-287).

I guess that some peripherials may use the upper bits A8..A15 of the address bus as a sort of 8-bit parameter.

Latrena

Thanks :-) Better emulate this behaviour anyway then. – Oxcart

The 'decreasing decoding complexity' hinted at by Phil is the most common use — using a single address line directly as device enable/disable is cheaper than logically combining several, and doesn't feel too wasteful when there are 16 of them. The keyboards on 8bit micros tend to exploit this behaviour to treat part of the port address as a sort of parameter, except that you're enabling or disabling keyboard lines with the address lines (so it's like a mask) and the result ends up being the logical AND of all affected lines (not on purpose, just because the board was open collector anyway). – Mccall

Answer 2

Some systems use the upper 8 bits as the address and the lower 8 bits as the parameter. The Amstrad CPC is the main example. This makes OUT (C),r almost the only usable instruction, although of course it now acts actually as OUT (B),r; C is often used as the parameter for convenience. The corollary is that OUT (n),A becomes almost completely useless, unless you happen to want to send 0x34 to port 0x34, etc.

Irreligious

Amstrad in fact has quite a few ports where all 16 bits have to be used. – Churinga

Answer 3

On ZX Spectrum the keyboard can be read only by reading from port 0xfe while the highest 8 address lines are selecting one of the 8 groups of 5 keys. For example, if you want to scan the keys Q, W, E, R, and T the upper 8 bits of the address bus have to be 0xfb:

    ld bc,#fbfe
    in a,(c)        ; reading from port 0xfe while upper 8 address lines are 0xfb

This is exactly the same as:

    ld a,#fb
    in a,(#fe)      ; reading from port 0xfe while upper 8 address lines are 0xfb

Some arcade machines from the 80s communicate with the additional hardware by outputting more than 8 bits at a time with one out instruction - the additional bits are read from the upper address lines.

Churinga