Reading, writing and testing memory on the HP 150 from the DEBUG prompt, plus the memory map and the 384K SRAM expansion card. Written against DEBUG from MS-DOS 2.x on the HP 150. Map is from Technical Manual 45625-90001, p.4-1/4-2.

Everything in DEBUG is hex. Never type an H suffix — 10 means sixteen.

The HP 150 is not an IBM PC. For testing, only write inside 40000-9FFFF.

# Start DEBUG with nothing loaded:

DEBUG

The prompt is a single -. A dash means DEBUG is waiting.

# Start DEBUG with a file loaded at CS:100:

DEBUG FILE.COM

# Quit back to DOS:

Q

Every address is SEGMENT:OFFSET. The physical address is (segment x 10h) + offset.

Leave the segment off and DEBUG picks one for you — DS for D, E, F, C, S, M and CS for U, A, G, T.

# Range as start plus END OFFSET, same segment:

D 4000:0000 7FFF

# Range as start plus L and a LENGTH:

D 4000:0000 L8000

Maximum length is L FFFF.

# A list — bytes and quoted text, mixed freely — for E, F and S:

E 4000:0000 01 02 FF "HELLO" 00

# Dump 128 bytes (8 lines) as hex and ASCII:

D 4000:0000

Each line is the address, 16 hex bytes, then the ASCII. A . means not printable.

# Dump exactly 16 bytes:

D 4000:0000 L10

# Carry on from where the last dump stopped:

D

# Find every AA in U10:

S 4000:0000 L8000 AA

Prints each address where the pattern was found.

# Find a text string:

S 4000:0000 L8000 "HP"

# Write three bytes, no questions asked:

E 4000:0000 55 AA 12

# Edit byte by byte, interactively:

E 4000:0000

DEBUG shows each byte in turn and waits.

# Fill all 32K of U10 with 55:

F 4000:0000 L8000 55

# Fill with a repeating pattern — 01 02 01 02 …:

F 4000:0000 L100 01 02

# Copy a block:

M 4000:0000 L100 4800:0000

# Compare two ranges and print every byte that differs:

C 4000:0000 L4000 4000:4000

No output at all means identical. Each line of output is a mismatch, in the order address 1, byte 1, byte 2, address 2.

# Sum and difference of two hex numbers:

H 4800 4000

Prints 8800 0800.

# Show all registers and the next instruction:

R

# Change AX — DEBUG prompts with a colon:

R AX

# Disassemble the firmware memory-sizing loop:

U F800:7A48

# Type in 8088 instructions at offset 100:

A 100

A blank line ends assembly.

# Run from CS:IP:

G

# Start at 100, stop at a breakpoint at 110:

G=100 110

# Run one instruction, then show the registers:

T

# Run five instructions:

T 5

# Read an I/O port:

I 40

# Write an I/O port:

O 40 FF

HP 150 I/O ports are not IBM PC ports. A port number out of a PC book hits something else here.

# Name the file to work on:

N TEST.COM

# Load the named file at CS:100:

L

# Write the named file back to disk:

W

W with sector numbers writes raw sectors and can wreck a floppy. Do not use W unless you mean it.

# List the commands:

?

Some versions only.

# 1. B800 is not video RAM here.

On the HP 150 it is the six system status LEDs. Writing to it changes the LEDs, not the screen.

# 2. BC00 is the saved configuration.

It is the CMOS configuration RAM. Writing to it changes the saved configuration. Stay inside 40000-9FFFF for anything that writes.

Twelve 32K chips, U10-U21, covering 40000-9FFFF.

# 1. Fit chips in fours, or the power-on test fails with 000C.

The power-on test wants expansion RAM in 128K steps — U10-U13, U14-U17, U18-U21. Any other count still works under DOS — U10 alone gave 288K in CHKDSK — but power-on stops here until you press Return:

Power-on test failed 000C

000C is two bits. Bit 3 — memory is not in whole 128K steps. Bit 2 — it then tests the missing 64K block and fails. Nothing is actually broken.

Each chip is 32K, so CHKDSK reads 256K + 32K x chips either way.

U10 shown throughout. Change the segment for other chips — see the card table above.

# Write a walking one, then a walking zero:

E 4000:0000 01 02 04 08 10 20 40 80 FE FD FB F7 EF DF BF 7F

# Read it back — it must match exactly as written:

D 4000:0000 L10

One value per address line. None may overwrite another.

# Write a different value at each power-of-two offset:

E 4000:0000 00
E 4000:0001 01
E 4000:0002 02
E 4000:0004 03
E 4000:0008 04
E 4000:0010 05
E 4000:0020 06
E 4000:0040 07
E 4000:0080 08
E 4000:0100 09
E 4000:0200 0A
E 4000:0400 0B
E 4000:0800 0C
E 4000:1000 0D
E 4000:2000 0E
E 4000:4000 0F

# Read each address back with L1 and check it holds its own value:

D 4000:0100 L1

Repeat for every offset in the list. A wrong value names the stuck or shorted address line.

Fill, compare the halves, repeat with the opposite pattern.

# Fill the chip with 55:

F 4000:0000 L8000 55

# Compare the two halves — no output means good:

C 4000:0000 L4000 4000:4000

# Fill the chip with AA:

F 4000:0000 L8000 AA

# Compare the two halves again:

C 4000:0000 L4000 4000:4000

A write to the next chip must not land in this one.

# Mark this chip:

E 4000:0000 11

# Mark the next chip:

E 4800:0000 22

# Check this chip — it must still be 11:

D 4000:0000 L1

# See what the bus returns where nothing answers — slot 2, not on this card:

D A000:0000 L20

# Check what DOS counted:

CHKDSK

The bytes total memory line is the number that matters.

Rule of thumb: read anywhere, write only in 40000-9FFFF, and fit the chips in fours.