[Sony Xperia T, TL, TX, V] Nagato Yuki:
Well, It is acceptable using Xperia TX even just 642MB usable for Android system and apps, until play the Japanese game call "Kaku-San-Sei Million Arthur" (拡散性ミリオンアーサー, Google Play) which sometime reboot my phone suddenly.

Once I make disable some apps, the frequency of crashing the system is greatly reduced even it could some time cause that happen. Then I guess the RAM usage of this game could be a bit larger then Android try to get more RAM for it, but it is not successful then the system crashed and reboot.

I recognized that the usable RAM on my Xperia is much less than Samsung GALAXY S II (i9100) and even GALAXY Tab 10.1. And look at the iomem and kmsg:

00500000-00500fff : msm_ssbi.0
00700000-007060ff : hdmi_msm_qfprom_addr
0080207c-0080207f : slimbus_slew_reg
0080207c-0080207f : msm_slim_ctrl
01a01000-01a01fff : msm_etb
01a03000-01a03fff : msm_tpiu
01a04000-01a04fff : msm_funnel
01a1c000-01a1dfff : msm_etm
03000000-0327ffff : wcnss_mmio
03204000-032040ff : pil_riva
04100000-04100fff : kgsl_2d0_reg_memory
04100000-04100fff : kgsl-2d0
04200000-04200fff : kgsl_2d1_reg_memory
04200000-04200fff : kgsl-2d1
04300000-0431ffff : kgsl_3d0_reg_memory
04300000-0431ffff : kgsl-3d0
04400000-044fffff : msm_vidc.0
04500000-045fffff : vfe32
04500000-045fffff : msm_vfe
04600000-046fffff : msm_gemini.0
04700000-047effff : mipi_dsi
04800000-048003ff : csid
04800000-048003ff : msm_csid
04800400-048007ff : csid
04800400-048007ff : msm_csid
04800800-04800bff : ispif
04800800-04800bff : msm_ispif
04800c00-04800fff : csiphy
04800c00-04800fff : msm_csiphy
04801000-048013ff : csiphy
04801000-048013ff : msm_csiphy
04801400-048017ff : csiphy
04801400-048017ff : msm_csiphy
04801800-04801bff : csid
04801800-04801bff : msm_csid
04a00000-04a00fff : hdmi_msm_hdmi_addr
04e00000-04efffff : msm_rotator.0
05100000-051effff : mdp
05300000-053fffff : semc_vpe.0
05300000-053fffff : semc_vpe
08800000-088000ff : pil_qdsp6v4.1
08900000-089000ff : pil_qdsp6v4.2
08b00000-08b00003 : pil_qdsp6v4.2
08b00000-08b00003 : pil_qdsp6v4.1
12180000-121807ff : core_mem
12180800-12181fff : sdcc_dml_addr
12182000-12183fff : sdcc_bam_addr
12240000-12240fff : bamdma_dma
12244000-12247fff : bamdma_bam
12400000-124007ff : core_mem
12400800-12401fff : sdcc_dml_addr
12402000-12403fff : sdcc_bam_addr
12480000-12480003 : gsbi_qup_i2c_addr
12480000-12480003 : qup_i2c
124a0000-124a0fff : qup_phys_addr
124a0000-124a0fff : qup_i2c
12500000-12501000 : msm_hsusb
12500000-12501000 : msm_otg
12500000-12500fff : msm_hsusb_host
12520000-12520fff : msm_hsic_host
12800000-12803fff : pipe_mem
16200000-16200003 : gsbi_qup_i2c_addr
16200000-16200003 : qup_i2c
16280000-16280fff : qup_phys_addr
16280000-16280fff : qup_i2c
16300000-16300003 : gsbi_qup_i2c_addr
16300000-16300003 : qup_i2c
16380000-16380fff : qup_phys_addr
16380000-16380fff : qup_i2c
16500000-16500003 : gsbi_resource
16500000-16500003 : msm_serial_hs
16540000-16540fff : uartdm_resource
18320000-1841ffff : msm_dmov
18500000-1850ffff : qce.0
18500000-1850ffff : qcrypto.0
1a000000-1a000fff : gsbi_resource
1a040000-1a040fff : uartdm_resource
1a040000-1a040fff : msm_serial_hsl
1a200000-1a200003 : gsbi_qup_i2c_addr
1a200000-1a200003 : qup_i2c
1a280000-1a280fff : qup_phys_addr
1a280000-1a280fff : qup_i2c
1a500000-1a5001ff : msm_rng.0
28080000-28081fff : slimbus_physical
28080000-28081fff : msm_slim_ctrl
28084000-28085fff : slimbus_bam_physical
28084000-28085fff : msm_slim_ctrl
28800000-288000ff : pil_qdsp6v4.0
2a03f720-2a04071f : tz_log.0
80200000-88cfffff : System RAM
80300000-80c7094f : Kernel text
80d00000-80fb358b : Kernel data
88d00000-88d00fff : ramdumpinfo
88d01000-88d04fff : amsslog
88de0000-88dfffff : ram_console
90000000-ab4fffff : System RAM
b9902000-bfffffff : System RAM

[ 0.000000] Memory: 139MB 437MB 102MB = 678MB total
[ 0.000000] Memory: [COLOR="Red"]656612k/891116k available[/COLOR], 38676k reserved, 155640K highmem
[ 0.000000] Virtual kernel memory layout:
[ 0.000000] vector : 0xffff0000 - 0xffff1000 ( 4 kB)
[ 0.000000] fixmap : 0xfff00000 - 0xfffe0000 ( 896 kB)
[ 0.000000] DMA : 0xff000000 - 0xffe00000 ( 14 MB)
[ 0.000000] vmalloc : 0xe1800000 - 0xfa000000 ( 392 MB)
[ 0.000000] lowmem : 0xc0000000 - 0xe1000000 ( 528 MB)
[ 0.000000] pkmap : 0xbfe00000 - 0xc0000000 ( 2 MB)
[ 0.000000] modules : 0xbf000000 - 0xbfe00000 ( 14 MB)
[ 0.000000] .init : 0xc0008000 - 0xc004a000 ( 264 kB)
[ 0.000000] .text : 0xc0100000 - 0xc0a70950 (9667 kB)
[ 0.000000] .data : 0xc0b00000 - 0xc0bca170 ( 809 kB)
[ 0.000000] .bss : 0xc0bca194 - 0xc0db358c (1957 kB)

comparing with GALAXY Tab 10.1

[ 0.000000] Memory: 511MB 230MB = 741MB total
[ 0.000000] Memory: [COLOR="Red"]738404k/738404k available[/COLOR], 309148k reserved, 104448K highmem
[ 0.000000] Virtual kernel memory layout:
[ 0.000000] vector : 0xffff0000 - 0xffff1000 ( 4 kB)
[ 0.000000] fixmap : 0xfff00000 - 0xfffe0000 ( 896 kB)
[ 0.000000] DMA : 0xff000000 - 0xffe00000 ( 14 MB)
[ 0.000000] vmalloc : 0xe8800000 - 0xf8000000 ( 248 MB)
[ 0.000000] lowmem : 0xc0000000 - 0xe8000000 ( 640 MB)
[ 0.000000] pkmap : 0xbfe00000 - 0xc0000000 ( 2 MB)
[ 0.000000] modules : 0xbf000000 - 0xbfe00000 ( 14 MB)
[ 0.000000] .text : 0xc0008000 - 0xc06d7cd4 (6976 kB)
[ 0.000000] .init : 0xc06d8000 - 0xc070d860 ( 215 kB)
[ 0.000000] .data : 0xc070e000 - 0xc079b160 ( 565 kB)
[ 0.000000] .bss : 0xc079b184 - 0xc0a5d230 (2825 kB)

Then is it possible to recompile the kernel to adjust the drivers to make fully use of the RAM?