==== Dave's ZX Spectrum Exploration Notes ====

The ZX Spectrum was a British computer produced by Sinclair Research in the 80's, The processor is a z80 produced by Zilog, still in production and has some interesting features, such as being a Von Neumann architecture it support self modifying code. The ROM on the ZX Spectrum includes a version of BASIC, but the device can also be programmed in assembler.

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== Registers ==

^ high 8bit ^ low 8bit ^
| a | f |
| b | c |
| d | e |
| h | l |
|  ix   |
|  iy   |

a is 8 bit accumulator
hl is sort of a 16 bit accumualtor (only destination for 16bit add)
b conventionally for loops : djnz is "decrement b, jump if not zero"

ld 'loads' right to left:

  * ld reg, 255   ; literal
  * ld reg, reg   ; register
  * ld (reg), reg ; dereference reg
  * ld reg, (reg) ; dereference reg
  * ld reg, (ix + 3) ; array lookup
 
16bit pairs, bc, de, hl, ix, iy

push/pop stack - 16bits, eg:

  * push hl

arithmetic: inc, dec, add, sub, adc, sbc

wierd block processing instructions, ld, as in load, i/d inc/dec, r for repeat. each one uses bc,de and hl - bc is counter, de destination, hl source.

    LD DE,0000h
    LD HL,4000h
    LD BC,0100h
    LDIR

copies 0x100 bytes of data from 0x4000 to 0x0000.

cpir is for searching memory, bc = counter, hl = source and a is the search value

    LD HL,0000h
    LD BC,0000h
    LD A,124
    CPIR