TI-85 Assembler Programming - The INC and DEC instructions

Moving data around using LD is useful, but programs have to change data in order to be interesting. The INC and DEC instructions are a common and simple way to change a value.

Here is an example of each:

     INC  A
     DEC  B
If A was 10, it is now 11. If B was 23, it is now 22. Pretty simple, right? INC increments, or adds one to, the value in a register. DEC decrements, or subtracts one from, the value in a register. It is just like saying A = A + 1 or A + 1 -> A. INC and DEC work on any single register, and are incredibly efficient.

The other way to use INC and DEC is to change the value of a memory location by using the HL register pair to store the address:

     LD   HL, $80DF   ; the memory location to change
     INC  (HL)        ; increment the data at that location
If the data at location $80DF was 16, then the new value at location $80DF will be 17. Incrementing data in memory is slower than incrementing registers, but it is still very fast. A typical LD operation takes anywhere from 7 to 16 clock cycles, while INC (HL) takes 7 and INC A takes only one!

The next lesson deals with using the code that is in the TI-85 ROM so that we can do screen output.

On to the next lesson: ROM calls

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This page created 12-27-95 by Greg Parker. Last update: 12-30-95