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OBD0 PM6 & PM7 RAM Address Locations

A comprehensive memory mapping reference sheet of internal RAM, bit flags, and XRAM registers for OBD0 PM6 and PM7 Honda ECUs.

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Adapted from pgmfi.org wiki

This reference sheet outlines the internal memory-mapped address spaces of the PM6 (USDM 1988–1991 Civic/CRX Si) and PM7 (JDM DOHC ZC) OBD0 Honda ECUs.

These ECUs utilize 8051-derivative microcontrollers, which partition memory into internal RAM, bit-addressable spaces, and external RAM (XRAM). This mapping is critical for reverse-engineering fuel/ignition maps, writing custom datalogging code, or modifying factory ROM routines.

8-Bit Internal RAM Locations (Bytes)

Below are the key 8-bit registers located in the primary internal RAM space:

Address Parameter / Short Name Notes & Functions
0x2C.1 MAP High CEL Bit flag; set to 1 if the MAP sensor reading exceeds safe parameters.
0x30 ECT Complement Bitwise complement of 0x31h (verifies ECT signal integrity).
0x31 ECT Engine Coolant Temperature sensor reading (mirrors XRAM 0x1004h).
0x32 IAT Intake Air Temperature sensor reading (mirrors XRAM 0x1005h).
0x33 IAT Complement Bitwise complement of 0x32h (verifies IAT signal integrity).
0x34 BARO Barometric Pressure (PA sensor on board).
0x36 MAP Manifold Absolute Pressure sensor reading.
0x37 MAP Complement Bitwise complement of 0x36h.
0x38 MAP* Locked/stored MAP value (updated when bit 0x79.4 is set).
0x39 TPS Throttle Position Sensor reading.
0x3A TPS (Backup) Secondary or filtered TPS reading.
0x3B RPM (8-bit) Simplified 8-bit engine speed calculation.
0x3D RPM Low Byte Low byte read from OKI 6260 coprocessor register 0x2004h.
0x3E RPM High Byte / Status High byte read from OKI 6260 coprocessor.
0x47 Limit RPM Low Low byte of RPM used for rev limit checks (copied from 0x3Dh).
0x48 Limit RPM High High byte of RPM used for rev limit checks (copied from 0x3Eh).
0x49 Prev RPM Low Historical low byte of engine speed (previous loop).
0x50 Prev RPM High Historical high byte of engine speed (previous loop).
0x51 Delta RPM Low Rate of RPM change (low byte). Calculated as 0x47 - 0x49.
0x52 Delta RPM High Rate of RPM change. Bit 0x17h indicates the sign (1 = negative).
0x53 VSS High High byte of 16-bit Vehicle Speed Sensor counter.
0x54 VSS Low Low byte of 16-bit Vehicle Speed Sensor counter.
0x5C 6260 Port 1 Data Loaded to OKI 6260 register 0x2001h if bit 0x52h is set.
0x5D 6260 Port 2 Data Loaded to OKI 6260 register 0x2002h if bit 0x52h is set.
0x5E 6260 Port 1 Data Loaded to OKI 6260 register 0x2001h if bit 0x53h is set.
0x5F 6260 Port 2 Data Loaded to OKI 6260 register 0x2002h if bit 0x53h is set.
0x60 6260 Port 1 Data Loaded to OKI 6260 register 0x2001h if bit 0x54h is set.
0x61 6260 Port 2 Data Loaded to OKI 6260 register 0x2002h if bit 0x54h is set.
0x6A Fuel Trim Add Primary short-term fuel trim adjustment.
0x6C VSS (8-bit) Simplified 8-bit vehicle speed value.
0x6F O2 Fuel Bias Closed-loop fueling correction factor based on O2 sensor.
0x70 O2 Fuel Bias (Filtered) Smoothed/integrated closed-loop fuel bias.
0xA2 Fuel Mult Low Low byte multiplier used to adjust fuel injector pulse width.
0xA3 Fuel Mult High High byte multiplier used to adjust fuel injector pulse width.

Internal RAM Bit Flags (Bit-Addressable)

Below are critical diagnostic and logic bits located in the bit-addressable internal RAM space:

Bit Address Flag Name Technical Note / Threshold
0x60h MAP CEL Flag Set to 1 when the manifold pressure is out of normal operational bounds.
0x17h RPM Delta Sign Sign bit for engine acceleration (0 = accelerating, 1 = decelerating).
0x2Eh RPM Limit Safe Set to 1 when current engine speed is below the active rev limit.
0x57.0 Ignition Coil 1 Firing indicator for coil phase 1.
0x57.1 Ignition Coil 2 Firing indicator for coil phase 2.
0x71.1 Idle Threshold Set to 1 if RPM is between ~890 and ~950 RPM (idle target zone).
0x71.2 VTEC RPM Cross Set to 1 if RPM exceeds ~5,400 to ~5,600 RPM.
0x71.3 Decel Vacuum Set to 1 if MAP is high-vacuum (~9.3 to ~8.7 in Hg / low load).
0x71.4 Medium Vacuum Set to 1 if MAP is mid-vacuum (~6.0 to ~5.0 in Hg).
0x79.4 Mid-RPM threshold Set to 1 if RPM is ~3400 to ~3650 RPM. Uses locked MAP 0x38h if set.
0x79.6 Low Load Threshold Set to 1 if MAP is ~6.0 to ~4.0 in Hg.
0x7A.4 High Load Threshold Set to 1 if MAP is ~8.1 to ~6.0 in Hg.

External RAM (XRAM) Locations

These addresses exist in the external static RAM chip (typically a standard 5128 SRAM chip mapped via the MOVX command):

XRAM Address Parameter / Short Name Notes & Functions
0x012h VSS High Buffer Temporarily holds the VSS high byte read from the OKI 6260 coprocessor.
0x013h VSS Low Buffer Temporarily holds the VSS low byte read from the OKI 6260 coprocessor.
0x032.2 Cranking Speed Flag Set to 1 if engine speed is in the starting range (~540 to ~630 RPM).
0x032.3 High Load Fuel Cut Set to 1 if MAP is extremely high pressure (~2.5 to ~1.0 in Hg).
0x07Dh ADC Destination PTR Address pointer specifying where the raw ADC byte from 0x4001h is written.
0x0A1h Checksum Accumulator Holds the running checksum tally during ROM verification routines.
0x0A2h Checksum Pointer Holds the next high address byte (DPH) during ROM checksum loops.

Credits and source

Authors tungsten2k

Source Adapted from OBD0PM6PM7RAM Locations on pgmfi.org wiki. Licensed under CC BY-NC-SA 1.0.