HP 150 Memory Poking with DEBUG and CHKDSK

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.