Getting Started - Revision history

Trixter: /* .COM file defaults */

2022-06-28T22:14:46Z

‎.COM file defaults

2020-04-28T07:24:18Z

replaced the memory segment trick

2020-04-20T02:16:17Z

2020-04-20T02:15:23Z

Added "how to think like a sizecoder"

2020-04-20T02:08:26Z

2018-03-27T15:13:51Z

‎.COM file defaults

2018-03-27T15:07:30Z

‎.COM file defaults

2016-08-28T19:35:16Z

‎A sample framework: My bad, had wrong assembler settings

2016-08-28T05:46:27Z

‎A sample framework

2016-08-28T05:44:40Z

‎1-byte opcodes

@@ Line 43: / Line 43: @@
 flags=xx02
 </pre>
 Because .COM files only support 64K executables, <code>DS</code>, <code>ES</code>, and <code>SS</code> are all set to the same value as <code>CS</code>.  The rest can't be counted on for any specific value, except that <code>BP</code> is mostly 09??h so you can usually count on the high byte being <code>09h</code>.

@@ Line 279: / Line 279: @@
          mov     al,13h          ;AX=0000 at program start
          int     10h             ;init mode 13h
-         mov     es,bp           ;BP=09xx at program start; close enough to A000 ;-)
+         les     bx,[bx]         ;contains 0x9FFF at program start; close enough to 0xA000 ;-)
 mainloop:

@@ Line 284: / Line 284: @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;This is where you do your mega-amazing tiny program.
-;Write 8-bit values to A000:0000 to draw some pixels.
+;Write 8-bit values to ES:xxxx to draw some pixels.
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

@@ Line 268: / Line 268: @@
          ret                     ;.COM files can exit with RET
 </syntaxhighlight>
 == Where to go from here? ==

@@ Line 52: / Line 52: @@
 === Boot sector defaults ===
-Boot sector tinyprogs are occasionally explored, but The BIOS changes every register value as it executes before the boot sequence, so there's not much to count on other than what occurs directly before execution of the boot sector:
+Boot sector tinyprogs are occasionally explored, but the BIOS changes every register value as it executes before the boot sequence, so there's not much to count on other than what occurs directly before execution of the boot sector:
 * The boot sector is loaded at 0000:7C00
 * DL holds the drive number that was booted from, so if booted from a floppy disk in drive A:, it will be 00

@@ Line 35: / Line 35: @@
 BX=0000
 CX=00FF
-DX=Same as CS register
+DX=CS
 SI=0100
 DI=FFFE
 SP=FFFC (DOS child process) or FFFE (the default)
 </pre>

← Older revision		Revision as of 15:07, 27 March 2018
Line 44:		Line 44:

	Usually, the top stack contains <code>0000h</code>. That allows for using the one byte instruction <code>RET</code> to exit your program, since at location <code>CS:0000h</code> there is the value <code>20CDh</code> (= <code>int 20h</code>). See [https://en.wikipedia.org/wiki/Program_Segment_Prefix Program Segment Prefix].		Usually, the top stack contains <code>0000h</code>. That allows for using the one byte instruction <code>RET</code> to exit your program, since at location <code>CS:0000h</code> there is the value <code>20CDh</code> (= <code>int 20h</code>). See [https://en.wikipedia.org/wiki/Program_Segment_Prefix Program Segment Prefix].
		+
		+	Different DOS versions may differ slightly in the above. A full chart exists at http://www.fysnet.net/yourhelp.htm to help you check if your specific target has different defaults than the standard settings above.

	=== Boot sector defaults ===		=== Boot sector defaults ===

@@ Line 238: / Line 238: @@
 == A sample framework ==
-Want to just dive in and see what happens?  Here's a skeleton that sets up Mode 13h, loops until a keypress is detected, then exits.  Although it assembles to only 20 bytes, do not consider this fully and finally optimized; your program's structure may allow some of this to be reduced further.  But it's a good place to start:
+Want to just dive in and see what happens?  Here's a skeleton that sets up Mode 13h, loops until a keypress is detected, then exits.  Although it assembles to only 19 bytes, do not consider this fully and finally optimized; your program's structure may allow some of this to be reduced further.  But it's a good place to start:
 <syntaxhighlight lang=nasm>

@@ Line 57: / Line 57: @@
 === 1-byte opcodes ===
-The 80x86 family was originally a [https://en.wikipedia.org/wiki/Complex_instruction_set_computing CISC] design, a design philosophy that attempts to create instructions that perform multiple steps.  As such, there are some single instructions that perform complex actions, such as the string opcodes (<code>LODS</code>, <code>MOVS</code>, <code>CMPS</code>, <code>SCAS</code>).  In sizecoding, you are trying to perform as much work in as little space as possible, so it is helpful to know (or memorize!) every 1-byte instruction in the 80x86 family.  Here's a handy chart (segments and prefixes omitted):
+The 80x86 family was originally a [https://en.wikipedia.org/wiki/Complex_instruction_set_computing CISC] design, a design philosophy that attempts to create instructions that perform multiple steps.  As such, there are some single instructions that perform complex actions, such as the string opcodes (<code>LODS</code>, <code>MOVS</code>, <code>CMPS</code>, and <code>SCAS</code>).  In sizecoding, you are trying to perform as much work in as little space as possible, so it is helpful to know (or memorize!) every 1-byte instruction in the 80x86 family.  Here's a handy chart (segments and prefixes omitted):
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