TI-82 ASM Corner :: Programming Guide

This lesson will give you some more in depth information on the way you can access data: addressing methods. So what exactly does addressing mean? Well, take for example LD A,($1234). Here, the data comes from a memory address. With LD A,(HL) the data also comes from a memory address, but this is register indirect addressing. It happens through the HL register.

The data is in one of the Z-80 registers or registerpairs.

 Examples :

 INC    HL
 LD     A,B
 LD     E,L
 DEC    L

Immediate Addressing

8-bits data is stored in the memory immediately after the opcode for the instruction

 Examples :

 LD     L,15
 ADC    A,125
 SUB    20
 CP     5

Immediate extended addressing

Same as above, but the data is 16-bit this time

 Examples :

 LD     HL,$800C
 LD     IY,$4414
 LD     IX,$88B8

Extended Addressing

Immediately after the opcode, there is a data word. A call or a jump needs to be made to that address or data needs to be received from that address.

 Examples :

 JP     $0213
 LD     ($800C),A
 LD     HL,($8D24)
 LD     A,($8000)

One of the register couples contains the address of the data

 Examples :

 LD     B,(HL)
 LD     (DE),A
 LD     (HL),$0A
 RR     (HL)
 JP     (HL)

Indexed Addressing

This way looks a lot like Register Indirect Addressing, but the Z-80 adds a byte (signed) to the address first. It only works with IX and IY.

 Examples :

 SRL    (IY-10)
 SET    3,(IY+05)
 ADD    A,(IX+05)

Relative Addressing

To save time and memory, an address can sometimes be given with only 1 byte (signed). The real 2-byte address is then calculated starting from the Program Counter (Address=PC+2+offset).

 Examples :

 JR     10
 JR     NC,-3

Modified Page Zero Addressing

The only page 0 addressing instructions on the Z-80 are the RST instructions. The address is calculated by putting the most significant bytes to $00 and the least significant byte to the given one.

 Examples :

 RST    $10
 RST    $38

Implied Addressing

One or both of the operands are not given in the instruction.

 Examples :

 SUB    B
 EXX
 LDIR
 CPIR

Bit Addressing

The Z-80 contains some instructions to set, reset and test a bit in a byte.

 Examples :

 BIT    3,(IY+$05)
 SET    1,(HL)
 RES    5,D

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