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This port only exists as a distinct port on the TI-83 Plus Silver Edition, the TI-84 Plus, and the TI-84 Plus Silver Edition. On the standard TI-83 Plus, it acts as a shadow of port 01.
This port is protected, which means user programs cannot ordinarily write to it directly.


Port Number: 21h

Function: Flash Size / RAM Size

Read Values

  • Bits 0-1: Set flash chip size (see below)
  • Bits 4-5: Set RAM chip size (see below)

Write Values

  • Bits 0, 1, 4, 5: set new values for these bits.
  • Bits 2, 3, 6, 7: ignored


This port does not exist on the standard 83+. See port 2.

The value of this port is initialized by the boot code. On the TI-83+SE, TI-84+, and TI-84+SE, bit 0 can be used to determine the hardware type alone. On the TI-84+CSE, use bits 1 and 0. If want to run on the TI-83+SE through TI+84+CSE for some reason, check both bits. The OS likes to check both bits by doing AND 3.

Flash Chip Size

The lower nibble controls the flash chip type. See also 83Plus:Memory_Mapping. Flash pages have three different properties determined by this port: readable/censored, privileged/unprivileged, and mutable/immutable. The only censored page is the certificate page. Privileged pages can unlock flash via port 14. Immutable pages cannot be written to even when flash is unlocked.

Value    Chip Size    Boot Page(s)    Certificate    Privileged    Immutable   
0 1024 K 3F 3E 2C-2F & 3C-3F 2C-2F & 3F
1 2048 K 7F 7E 6C-6F & 7C-7F 6C-6F & 7F
2 4096 K FF FE EC-EF & FC-FF EC-EF & FF
3 8192 K 1FF 1FE 1EC-1EF & 1FC-1FF    1EC-1EF & 1FF

RAM Chip Size

The TI-83+SE had an external RAM chip. This port therefore informs the ASIC what size RAM chip is installed.

Value    Chip Size   
0 32K
1 64 K
2 128 K
3 256 K

The actual execution permissions of RAM pages are masked by this port. Because TI sets it 0 (instead of 2), the ASIC thinks there's 32 K of RAM, and allows execution on all odd-numbered pages, instead of just 81h like TI intended. (If you set this to 1, 2, or 3, execution is also allowed on the first 1024 bytes of each page immediately following an allowed page because TI screwed up on port 26h.) If you ever attempt to execute code from a non-executable page, the calculator resets.


	in a,(2)
	ld b,a
	and 80h
	jr z,TI83p_BE
	in a,(21h)
	and 3
	jr z,TI84p_BE
	bit 5,b
	jr z,TI83p_SE
	; calculator is an 84+ SE

	; calculator is an 84+ BE

	; calculator is an 83+ SE

	; calculator is an 83+ BE