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references/TimothyLottesBlog/20150426_-_Source-Less_Programming__5.md

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+# 20150426 - Source-Less Programming : 5
+
+**Source:** https://refined-github-html-preview.kidonng.workers.dev/gomson/TimothyLottes.github.io/raw/refs/heads/master/20150426.html
+
+# 20150426 - Source-Less Programming : 5
+
+  
+**Boot Loader Bring-up**
+  
+Managed to get the boot loader done, which includes the following steps,  
+  
+(1.) Move the stack seg:pointer (since next step overwrites it).  
+(2.) Use BIOS to read the other 62 512-byte sectors for the first track.  
+(3.) Use BIOS to switch to 80x50 text mode and load custom character glyphs.  
+(4.) Use BIOS to set EGA text palette to 0-15 with 0 for overscan.  
+(5.) Program VGA palette registers for those 16 colors.  
+(6.) Use BIOS to enable A20.  
+(7.) Turn off interrupts, and relocate the image's 63 sectors to zero.  
+(8.) Load zero entry IDT, minimal 3 entry GDT.  
+(9.) Enable protected mode and jump to the 3rd sector.  
+  
+The 2nd 512-byte sector contains the 8x8 character bitmaps for the first 64 characters.
+The majority of the time was spent making a nice font,
+getting colors the way I wanted,
+and prototyping editor look and feel (without building it).  
+  
+Didn't feel like fully hand assembling 16-bit x86 machine code for the boot loader,
+so I used NASM and hexdump to accellerate the process
+(to provide machine code I could pad out to 32-bit alignment).
+Also wrote a quick C based tool to bootstrap the process of building the loader.
+Something which would enable me to easily build out an annotated image,
+and show a print out in the console of what I'd be seeing in the editor.
+Here is a shot of a bit of the scratch C code I used to make the font,  
+  
+![](20150426-A.png)
+  
+Here is a shot in QEMU of the loader displaying the font,  
+  
+![](20150426-B.png)
+  
+And another shot from QEMU showing the pallet,  
+  
+![](20150426-C.png)
+  
+**What the Current Annotated Image Looks Like**
+  
+Below is a shot captured from the terminal window output of the C tool.
+I'm using 3 cache lines for the loader code.  
+  
+![](20150426-D.png)
+  
+Grey lines separate the 512-byte sectors.
+Memory address on the left in grey.
+Each pair of lines shows half a x86 cacheline.
+The blue to white shows the 5 character/word annotation strings
+(now using the extra 2 bits of the label for color).
+The red hex show the image data.
+Not using {GET,ABS,REL} tagged words in this part,
+so everything in the bootloader is just hand assembled 16-bit machine code,
+and this is not representative of what the rest of the system will look like.
+The rest of the system will have {GET opcode} followed by {HEX} or {ABS}
+for opcode immediates (easy to write).
+The 16-bit code is {HEX} mixed opcode and immediates, quite a bit different (hard to write).  
+  
+Some hints on the annotations,  
+  
+Everything is in base 16.
+AX is TOP so I don't bother with "A=9000" (which wouldn't fit anyway),
+instead I just write "9000" (the A= is implied).
+The "!" means store so "SSSP!" is storing TOP (or AX) into both SS and SP.
+The "B=200" means BX=200h.
+In this 16-bit x86 case I use 3E to pad out opcodes to 32-bit.
+The "X" = SI, "Y" = DI, "F" = BP.  
+  
+**Next Step**
+  
+Ground work is done,
+next step is to bring up the opcode dictionary for {GET} words,
+then write a little IDE driver to get access to load the rest of the image,
+and to be able to save in the editor.
+After that, write the drawing code for the editor,
+then a mini PS/2 driver for the input,
+then write editor input handling.
+Then I have a full OS ready to start on a real machine.