16-bit operations

Perform operations on 16 bit data with an 8 bit machine.

This section uses a concept called accumulator staging. Similar to exchanging, accumulator staging provides the writeback two cycles later which allows the programmer to do a single-cycle instruction before the accumulator is automatically reflected as the register again.

fetch	fetch	decode	decode	execute	writeback
xch ra		load ra		write acc to reg	written to acc

From the instruction above, this behaviour is changed to:

fetch	fetch	decode	decode	execute	writeback
stg ra		load ra			written to acc
(e.g.) bsl 1				barrel shift left	written to acc, write to reg (from staging)

A hazard is that the second next instruction after stg cannot be a register store.

1 16-bit accumulative addition

xy += lh (4 bytes)

stg   rx    ; low byte
add   rl    ; low byte offset
stg   ry    ; high byte
addc  rh    ; high byte offset

2 16-bit increment

xy += 1 (3 bytes)

inc   rx    ; write back rx
stg   ry    ; stages/writes back ry
addc  zr    ; propagate carry (adds zero)

3 16-bit add immediate with 8-bit constant

xy + imml (5 bytes)

stg   rx    ; stages/writes back rx
addi  0x##  ; adds 8 bit to rx
stg   ry    ; stages/writes back ry
addc  zr    ; 3 instructions ago, but imm doesn't change acc, so valid

4 16-bit copy

lh = xy (4 bytes)

ast   rx
rst   rl
ast   ry
rst   rh