'===========================================================================
' Subject: RELATING ASSEMBLY TO BASIC         Date: 06-25-98 (00:00)       
'  Author: Matt Gulden                        Code: Text                   
'  Origin: www.thrillhaus.com/basic.html    Packet: FAQS.ABC
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.oOo. Relating Assembly Language to BASIC
by Matt Gulden (aka Folter) - mattg@thrillhaus.com
==================================================

This article is intended to make learning asm (assembly
language) easier by using references to BASIC.

Now, let's get to the code!

  <<<<< The DEF SEG and POKE statements >>>>>
  Specifies memory segement that POKEs will use.
  Syntax: DEF SEG = segment: POKE location, value

Example:
DEF SEG = &HA000
POKE 0, 15

Example in ASM:

; We need to point ES to the segment of memory, but can't do it directly.
; This is the equivalent of DEF SEG
mov ax, A000h
mov es, ax

; This sets DI to 0, which is the location in memory we want to poke to.
xor di, di

; AL will hold the value
mov al, 15d

; STOSB moves AL to location DI at segment ES, and increments DI once, so if you did stosb
; again, it would poke to the location 1, instead of 0.
stosb

Now, there's also, STOSW, which would allow you to poke an entire word to memory,
and it increments DI twice.

  <<<<< The SCREEN statement >>>>>
  Calls a certain screen mode.
  Syntax: screen mode (where mode is a screen mode)

Example:
SCREEN 13

Example in ASM:

; AX will contain the mode we want to call. Interrupt 10 will call the mode specified in AX.
mov ax, 0013h
int 10h

  <<<<< The OUT statement >>>>>
  Outputs a byte or a word to a certain port.
  Syntax: OUT port, value

Example:
OUT 968, 255

Example in ASM:

; Set port and value
mov dx, 968d
mov al, 255d
out dx, al

; Now that would only output a byte. If you wanted to output a word, you can do this:
mov dx, 968d
mov ax, 255d
out dx, ax

; You can only use AX and AL with the ASM out statement.

-Matt Gulden