Last Updated or created 2023-05-24

When selling a large part of my computer collection I kept a few odd pieces.

Amongst those was a 8088 DIY machine.

It is a 50PTX3 motherboard with a 8088 compatible CPU

• Power light (Not LED) and connected to power adaptor, not motherboard
• Reset button??? not connected
• Power switch
• Mid center, 5v gnd and 12v
• 5.25″ drive not connected

Tested the power adapter first, a nice 5V and 12V.
Then I plugged the power in the Motherboard add plugged a test ISA card in the slot.
After turning the machine on I saw the Address leds flashing

A NEC D70108C from 1984, which is 8088 pin compatible with Intel 8088 but faster, and has some extra functionality.
The empty socket is for the 8087 Co-processor.

Everything pretty dirty

Adding an 8bit Isa hercules/CGA card.

It starts! .. But there is no Floppy controller (yet)

I’ve dumped the Bios to a file and used a hexeditor to play around.
So that’s why there is a bad checksum.

Installing a ZIF socket (Zero Insertion Force) to make things easier to modify.

Burned GlaBios on the Eprom and now I can continue to play around.

So why? Why this all ..

I want to play around with old 8088 assembly code again, but not as I did before using a Dos machine, but hardcoded into Eproms.
I’ve got 8 banks for ROMs and the source code for GlaBios is available.

In the past Edk and me wrote a boot demo, so it was not utilizing Ms-Dos functions. Maybe i can get some graphical and sound stuff working straight from the Bios.

Some commands:

# Dump the bios to file
minipro -w original.rom -p AT2764A@DIP28

xxd and hexdump to view the dump

I've used ghex to alter the ROM

# Write a new bios to a 28C64 (same Eeprom i've used for the C64 Cartridges)
minipro -w /home/henri/Downloads/MYROM -p AT28C64
 

GlaBios source code:
https://github.com/640-KB/GLaBIOS/blob/main/src/GLABIOS.ASM

I was planning to disassemble the Phoenix Bios, but it’s quite hard to differentiate between code and data, there are no named pointers and you have to interpret every line of code.

So GlaBios it is ..

First code to look at:

This is after the whole post reset.
There is a reset pointer at ffff:fffe
Which points to the bootstrap routine, which ends in below machine code.
I'm going to plug my own code over here.

(See the funny remark about Monster as being displayed in one of above pictures)

;----------------------------------------------------------------------------;
; INT 18 - Unbootable IPL
;----------------------------------------------------------------------------;
; Display a disk boot failure message and wait for a key to cold reboot.
;
; This may be re-vectored to ROM BASIC, if present.
;
; Size: 18 bytes
;----------------------------------------------------------------------------;
INT_18 PROC
		ASSUME DS:_BDA_ABS
	PRINT_SZ  BOOT_FAIL			; print boot failure string
	XOR	AX, AX				; AH = 0 (wait for key)
	MOV	DS, AX				; DS = 0000
	MOV	WARM_FLAG_ABS, AX			; do a cold boot
	INT	16H					; wait for key press
	JMP	BOOT					; reboot
INT_18 ENDP

BOOT ENDP

;----------------------------------------------------------------------------;
;
; END OF BIOS POST/BOOTSTRAP
;
;----------------------------------------------------------------------------;

		ASSUME DS:_BDA
STRINGS PROC
;----------------------------------------------------------------------------;
; Banner Strings
;
BANNER_STRINGS PROC

					IF POST_GLADOS EQ 1
BOOT_BEGIN		DB	CR, LF
			DB	'Starting GLaDOS...'
NL2_Z			DB	LF				; two NL's, null term'd
					ENDIF
NL_Z			DB	CR, LF, 0			; one NL, null term'd
BOOT_FAIL		DB	'Disk Boot Fail.'
			DB	' You monster.'
NL2_ANY_KEY		DB	LF
NL_ANY_KEY		DB	CR, LF
ANY_KEY		DB	'Press the Any Key'
			DB	'...'

Assembly stuff

I’ve got Dosbox installed on my machine.

git clone https://github.com/640-KB/GLaBIOS.git
I copied MASM.EXE and LINK.EXE in the GLaBios src directory.

edit make.bat

change
MASM GLABIOS;
into
MASM /DVER_DATE=”05/24/23″ /DARCH_TYPE=”T” /DCPU_TYPE=”V” GLABIOS;

start dosbox

mount c: /home/henri/projects/

c:

cd glabios/src

make.bat