lesson1

LESSON 1:

To make an ASM program you will need the foloowing files:

ASM86v?? (found on ticalc.org)
TASM (also found on ticalc.org)
TASM80.TAB (notice a patern)

After you have all the programming utilities you need, you are almost ready to start your first program. This is what every ASM program will look like:

#include "asm86.h"         ;I use Asm86 to compile, so I include this
#include "ti86asm.inc"     ;this is TI's include file
.org _asm_exec_ram          ;starting point for all asm progs, $D748
   ; all the ASM goes in here
.end

The most important thing you need to understand is what a register is, and how to use it. A register is a 16bit (2 byte) memory location, found on the CPU itself. On the Z80 you have the following important Registers:

AF
BC
DE
HL
IX
IY
(there are others, but we won't use them in my examples, I don't even unstand them all)

These registers can be broken up into TWO 8bit (1 byte) registers each. (ie A,F,B,C,D,E,L,..) Of these 8 bit registers, A is the most important. Register A is what is called the Accumulator, some calls will only work on A (sub,or,xor,and,...). The A register is grouped with the F register.. Why? well F stands for Flags, and it contains the follwing flags (sign,zero,half carry,parity,add/subject,carry)

These are the instructions that will be covered in this lesson: LD ADD/SUB INC/DEC

LD
The first thing you will want to do is to load a value into a register. This is done with the LD instruction. Here are some examples:

ld a,0    ; loads the value 0 into register a
ld b,2    ; loads the value 2 into register b
ld de,257 ; loads the value 257 into register de
          ;(same as loading 1 into d and 1 into e)
ld d,$0A ; NOTE $8A represents a HEX number,
          ; %00100100 represents a BIN number,
          ; 52 just a decimal number.
          ; this loads $0A into d, $0A is the same as 10
ld a,d    ; loads the current value of d into a (in this case 10)

NOTE: An 8-bit regiser can only hold the values from 0-255 (%00000000-%11111111), but a 16 bit register can hold the values 0-65535.

The register HL, is primarly used for ADDRESSING, this means it usually points to another memory location. The video Memory is located at $FC00, so to have hl "point" to the video memory you use the command:

ld hl,$FC00 ;loads the value $FC00 into register hl

Now, to copy a value into the memory location that HL is pointing to, we do somthing called inderect addressing. This is accomplished by placing parens around the register name.

ld a,%10101010    ;loads the value %10101010 into reg. a
ld (hl),a         ;loads the value %10101010 into the
                  ;memory location that hl "points" to
                  ;the value of HL is $fc00 therefore
                  ;the value %10101010 is loaded into
                  ;memory location $fc00, which happens
                  ;to be the video memory :)
                  ;IT DOES NOT CHANGE THE VALUE OF HL!!
ld a,(hl)         ;similiarly this loads the value at
                  ;mem location $fc00 into the reg. a

back to list of instructions

ADD/SUB
The next thing to learn, is how to add and subtract from a register. To do this we use the indtructions ADD and SUB.

add a,##
add hl,ss (where ss = bc,de,hl, or sp)
add ix,pp (where pp = bc,de,ix, or sp)
add iy,rr (where rr = bc,de,iy, or sp)

These are the only ways that ADD can be used. ex:

ld a,8 ;a=8
add a,10 ;a=a+10 a=18
ld hl,$FC00 ;hl = $FC00
ld bc,$00BB ;bc = $00BB
add hl,FCBB ;hl=hl+bc hl = $fcbb

In order to add anything to the other registers, you must do it inderectly:

ld b,8      ;b=8
ld a,b      ;a=b
add a,5     ;a='b+5'
ld b,a      ;b='b+5'
;or
ld bc,46    ;bc=46
ld h,b      ;you can't do 'ld hl,bc'
ld l,c      ;
ld bc,52    ;
add hl,bc   ;hl = bc+52
ld b,h      ;.
ld c,l      ;bc = bc+52

Now you know how to add, what about subtracting?

sub ## ; a=a-##

That's it! you can only SUB from a (the accumulator), therefore all other subratcions must be made inderectly. Here are some examples:

ld a,16    ;a=16
sub 5      ;a=a-5, a=11
ld b,65    ;b=65
ld a,b     ;a=65
sub 6      ;a=65-6, a=59
ld b,a     ;b=59

ADD and SUB let you add or subtract any number, however if you only want to add or subract the value 1 then you can use INC/DEC

back to list of instructions

INC/DEC

inc r ;(where r=a,b,c,d,e,h,or l)
inc ss ;(where ss=bc,de,hl, or sp)
dec r
dec ss

These are the ony cases we will use. Here are some examples:

ld a,5    ;a=5
inc a     ;a=a+1, a=6
ld b,a    ;b=a, b=6
dec b     ;b=b-1, b=5
ld bc,568 ;bc=568
inc bc    ;bc=bc+1, bc=569
inc bc    ;bc=bc+1, bc=570

back to list of instructions