copy-instruction.txt

Three arguments: dest, src, mode
Mode is a word of three bytes, separated into three one-byte sub-args

The sub-args are:
- High byte: mode arg
- Mid / low bytes are a 16-bit limit arg
- (Some end modes only use the low byte)

Mode arg is three fields: 
- how the src is interpreted / incremented
- how the dest is interpreted / incremented
- how the end of the loop is triggered

src modes: (000000xx)
- src is address, increment (00)
- src is address, decrement (01)
- src is address, constant (10)
- src is literal (11)

dest modes: (0000xx00)
- dest is address, increment (00)
- dest is address, decrement (01)
- dest is address, constant (10)
- dest is ignored, do not copy (11)

end modes: (00xx0000)
- Count: iterate limit times (00)
- Until: iterate until src equals end arg (low byte) (01)
- While-greater: iterate while src is greater than end arg (low byte) (10)
- While-less: iterate while src is less than end arg (low byte) (11)

Return values:
- Mode is consumed
- Both src and dest args are left on the stack...
- But in the state they were in when the end test failed

Examples:
- Copy 16 bytes from 0x1000 to 0x1000:
    push 0x2000
    push 0x1000
    copy 16 ; leaves 0x2010, 0x1010 on the stack; last byte copied was 0x100f
- Copy from 0x1000 to 0x2000 until a null terminator:
    push 0x2000
    push 0x1000
    copy 0x100000
    ; Or until a space char (32):
    copy 0x100020
- Find the first 0 after 0x1000 (strlen):
    push 0x1000 ; Don't need a dest since it'll be ignored anyway
    copy 0x1c0000 ; leaves the address of the 0 on the stack
- Clear an array of 64 bytes after 0x1000 (memset)
    push 0x1000
    push 32 ; what we'll clear it to
    copy 0x030040 ; end mode: count, dest mode: increment, src mode: literal, mode arg: 64
- Copy each byte of a null-termed string to 0x41 (puts):
    push 0x41
    push 0x1000
    copy 0x180000 ; end mode: until, dest mode: constant, src mode: increment, mode arg: 0