New Apple Silicon M4 and M5 HiDPI Limitation on 4K External Displays

Starting with the M4 and including the new M5 generations of Apple Silicon, macOS no longer offers or allows full-resolution HiDPI 4k modes for external displays.

The maximum HiDPI mode available on a 3840x2160 panel is now just 3360x1890 (with a 6720x3780, instead of 7680x4320 backing store) - M2/M3 machines did not have this limitation.

With this regression Apple is leaving users to choose between:

• Full screen real estate at 4k (3840x2160) with blurry text due to HiDPI being disabled.
• Reduced screen real estate at 3.3k (3360x1890) with sharp text (HiDPI) but significantly less usable working space, and macOS’s UI looking ridiculously oversized.

This does not appear to be a hardware limitation

The DCP-reported capabilities are byte-for-byte identical between M2 Max and M5 Max. The limitation is in the GPU driver’s (AppleDisplayCrossbar) mode generation policy, which sits between the DCP and WindowServer. On M4/M5, this driver caps the HiDPI backing store at approximately 1.75x the native resolution, rather than the 2.0x needed for full 3840x2160 HiDPI. The M5 Max hardware supports 8K (7680x4320) at 60Hz natively - the GPU driver simply won’t allocate the framebuffer.

Environment and Test Setup

| Property | M5 Max (affected) | M2 Max (working) | |---|---|---| | Chip | Apple M5 Max | Apple M2 Max | | Model ID | Mac17,6 | Mac14,6 | | GPU Cores | 40 | 38 | | macOS | 26.4 (25E246) | 26.4 (25E246) | | Display | LG HDR 4K 32UN880 | LG HDR 4K 32UN880 | | Native Resolution | 3840x2160 | 3840x2160 | | Connection | USB-C/Thunderbolt, HBR3 (8.1 Gbps), 4 lanes | USB-C/Thunderbolt, HBR3 (8.1 Gbps), 4 lanes | | Max HiDPI Mode | 3360x1890 | 3840x2160 |

Both machines report identical DCP (Display Coprocessor) parameters for the LG display:

MaxActivePixelRate = 497,664,000
MaxTotalPixelRate = 537,600,000
MaxW = 3840
MaxH = 2160
MaxBpc = 10

The M5 Max officially supports “one external display up to 8K (7680x4320) at 60Hz” per Apple’s specifications. The hardware is unquestionably capable.

Diagnosis and Troubleshooting

Display Override Plist (scale-resolutions)

What: Wrote a display override plist to /Library/Displays/Contents/Resources/Overrides/DisplayVendorID-1e6d/DisplayProductID-7750 containing scale-resolutions entries for 7680x4320 HiDPI.

Result: No effect on M5 Max. The identical plist produces 3840x2160 HiDPI on M2 Max. WindowServer on M5 Max refuses to enumerate the mode regardless of plist content.

EDID Patching (Software Override)

What: Wrote a patched EDID into the override plist’s IODisplayEDID key with boosted range limits.

Result: No effect. The DCP reads EDID from the physical display over DDC/AUX, not from the override plist. The patched EDID only affects the macOS software layer, which is not where the limitation is enforced on M5 Max.

IOKit Registry Override (DisplayHints)

What: Attempted to modify the DCP’s DisplayHints dictionary and ConnectionMapping array directly in the IOKit registry.

Result: The DCP driver explicitly rejects userspace property writes with kIOReturnUnsupported. These properties are owned by the kernel-level AppleDisplayCrossbar driver and cannot be modified from userspace.

Hypothesis: Where the Limit Is Applied

According to BetterDisplay developer waydabber: “Generally 3840x2160 HiDPI is not available with any M4 generation Mac on non-8K displays due to the new dynamic nature of how the system allocates resources. The system allocates as low framebuffer size as possible, anticipating further displays to be connected and saving room for those.”

This aligns with our findings. The M4/M5 DCP firmware implements a conservative framebuffer pre-allocation strategy that caps the backing store based on the display's reported native resolution.